CHAPTER I ORGANIC BEHAVIOUR
I.—Behaviour in GeneralWe commonly use the word “behaviour” with a wide range of
meaning. We speak of the behaviour of troops in the field, of the
prisoner at the bar, of a dandy in the ball-room. But the chemist
and the physicist often speak of the behaviour of atoms and
molecules, or that of a gas under changing conditions of
temperature and pressure. The geologist tells us that a glacier
behaves in many respects like a river, and discusses how the crust
of the earth behaves under the stresses to which it is subjected.
Weather-wise people comment on the behaviour of the mercury in a
barometer as a storm approaches. Instances of a similar usage need
not be multiplied. Frequently employed with a moral significance,
the word is at least occasionally used in a wider and more
comprehensive sense. When Mary, the nurse, returns with the little
Miss Smiths from Master Brown’s birthday party, she is narrowly
questioned as to their behaviour; but meanwhile their father, the
professor, has been discoursing to his students on the behaviour of
iron filings in the magnetic field; and his son Jack, of
H.M.S.Blunderer, entertains
his elder sisters with a graphic description of the behaviour of a
first-class battle-ship in a heavy sea.The word will be employed in the following pages in a wide
and comprehensive sense. We shall have to consider, not only the
kind of animal behaviour which implies intelligence, sometimes of a
high order; not only such behaviour as animal play and courtship,
which suggests emotional attributes; but also forms of behaviour
which, if not unconscious, seem to lack conscious guidance and
control. We shall deal mainly with the behaviour of the animal as a
whole, but also incidentally with that of its constituent
particles, or cells; and we shall not hesitate to cite (in a
parenthetic section) some episodes of plant life as examples of
organic behaviour.Thus broadly used, the term in all cases indicates and draws
attention to the reaction of that which we speak of as behaving, in
response to certain surrounding conditions or circumstances which
evoke the behaviour. The middy would not talk of the behaviour of
his ship as she lay at anchor in Portland harbour; the word is only
applicable when there is action and reaction as the vessel ploughs
through a heavy sea, or when she answers to the helm. Apart from
gravitation the glacier and the river would not “behave in a
similar manner.” Only under the conditions comprised under the term
“magnetic field” do iron filings exhibit certain peculiarities of
behaviour. And so, also, in other cases. The behaviour of cells is
evoked under given organic or external conditions; instinctive,
intelligent, and emotional behaviour are called forth in response
to those circumstances which exercise a constraining influence at
the moment of action.It is therefore necessary, in a discussion of animal
behaviour, that we should endeavour to realize, as far as possible,
in every case, first, the nature of the animal under consideration;
secondly, the conditions under which it is placed; thirdly, the
manner in which the response is called forth by the circumstances,
and fourthly, how far the behaviour adequately meets the essential
conditions of the situation.II.—Behaviour of
CellsFrom what has already been said it may be inferred that our
use of the term “behaviour” neither implies nor excludes the
presence of consciousness. Few are prepared to contend that the
iron filings in a magnetic field consciously group themselves in
definite and symmetrical patterns, or that sand grains on a
vibrating plate assemble along certain nodal lines because they are
conscious of the effects of the bow by which the plate is set in
sounding vibration. But where organic response falls under our
observation, no matter how simple and direct that response may be,
there is a natural tendency to suppose that the behaviour is
conscious; and where the response is less simple and more indirect,
this tendency is so strengthened as to give rise to a state of mind
bordering on, or actually reaching, conviction. Nor is this
surprising: for, in the first place, organic responses, even the
simplest, are less obviously and directly related to the interplay
of surrounding circumstances; and, in the second place, they are
more obviously in relation to some purpose in the sense that they
directly or indirectly contribute to the maintenance of life or the
furtherance of well-being. Now where behaviour is complex and
subserves an end which we can note and name, there arises the
supposition that it may well be of the same nature as our own
complex and conscious behaviour.Take for example the behaviour of the Slipper-animalcule,
Paramecium, one of the minute creatures known to zoologists as
Protozoa. The whole animal is constituted by a single cell,
somewhat less than one-hundredth of an inch in length, the form and
behaviour of which may be readily studied under the microscope.
Thousands may be obtained from water in which some hay has been
allowed to rot. The surface of the Paramecium is covered with
waving hair-like cilia, by which it is propelled through the water,
while stiffer hairs may be shot out from the surface at any point
where there is a local source of irritation, as indicated at the
top of the accompanying figure. Two little sacs expand and
contract, and serve to drain off water and waste products from the
substance of the cell. Food is taken in at the end of the funnel,
shown in the lower part of the figure. The cilia here work in such
manner as to drive the particles into and down the tube, and on
reaching its inner end these particles burst through into the
semi-fluid substance, and circulate therein. Just above the funnel
there are two bean-like bodies, the larger of which is known as the
macronucleus, the smaller as the micronucleus.Fig. 1.—Paramecium.The process of multiplication is by “fission,” or the
division of each Paramecium into two similar animalcules. Not
infrequently, however, two Paramecia may be seen to approach each
other and come together, funnel to funnel; and in each the nuclei
undergo curious changes. The macronucleus breaks up, and is
scattered. The micronucleus in each divides into four portions, of
which three break up and disappear; while the fourth again divides
into two parts, one to be retained and the other to be exchanged
for the similar micronuclear product of the other Paramecium. The
retained portion and that received in exchange then unite to form a
new micronucleus. M. Maupas concludes from his careful observations
that, in the absence of such “conjugation” in the mid-period of
life, Paramecia pass into a state of senility which ends in
decrepitude and death. If this be so, conjugation is in them
necessary for the continuance of a healthy race.Here we have what a zoologist would describe as a specialized
mode of behaviour of the nuclei; and we have also the behaviour of
the minute creatures (which contain the nuclei) as they approach
each other and come together in conjugation. Can one wonder that
the latter, at any rate, has been regarded as an example of
conscious procedure? In truth we do not know in what manner and by
what subtle influences the Paramecia are drawn together in
conjugation. But it is scarcely logical to base on such ignorance
any positive assertion as to conscious attraction. It is better to
confess that here is a piece of organic behaviour, the exact
conditions of which are at present unexplained.We may take from the writings[1]of Dr. H. S. Jennings, of Harvard, some account of other
modes of behaviour among Paramecia. They largely feed upon clotted
masses of bacteria. If a number are placed upon a glass slip,
together with a small bacterial clot, they will be seen to
congregate around the clot and to feed upon it. All apparently
press in so as to reach it, or get as near it as possible. And if a
number be placed on another slide without any clot, they soon
collect in groups in one or more regions, as in Fig. 2, III. It
appears as if they were actuated by some social impulse leading
them to crowd together and shun isolated positions. Nay, more; it
seems as if, after thus collecting and crowding in to some centre
of interest, the attractive influence gradually waned; the group
spreads, and the Paramecia are less densely packed; the assembly
scatters more and more, but still seems to be retained by an
invisible boundary beyond which the little creatures do not
pass.Fig. 2.—Behaviour of Paramecia (after Jennings).Furthermore, if kept in a jar, the Paramecia crowd up towards
the surface where the bacteria clots are floating; and if, beneath
the cover glass of a slip on which they are under microscopic
examination, a drop of liquid be introduced through a very fine
tube, they will seem either to be attracted to it, as in Fig. 2,
I., or repelled from it, as in Fig. 2, II., according to its
nature. From alkaline liquids they are repelled; to slightly acid
drops they are attracted, unless the acidity be too pungent. Heat
and cold are alike repellent, and even a drop of pure distilled
water forms an area into which the Paramecia do not
enter.With such facts before him, the incautious observer may be
led to the conclusion that Paramecia are not only conscious, but
endowed with intelligence and volition. Even M. Binet,[2]who occupies a position which should lead him to exercise
more caution, tells us that there is not a single infusorian which
cannot be frightened, and does not manifest its fear by rapid
flight; he speaks of some of these unicellular animals as “endowed
with memory and volition,” and possessed of “instinct of great
precision;” and he describes the following stages:—
“ (1) The perception of an external object;
“ (2) The choice made between a number of objects;
“ (3) The perception of their position in space;
“ (4) Movements calculated either to approach the body and
seize it, or to flee from it.”But when we seem to have grasped his point of view, when we
have catalogued the memory, fear, instinct, perception, choice and
volition, the whole intelligent edifice crumbles; for we are told
that “we are not in a position to determine whether these various
acts are accompanied by consciousness, or whether they follow as
simple physiological processes.” To most of us fear, memory,
choice, volition, imply something more than simple physiological
processes; they imply not only consciousness, but highly elaborated
consciousness.Dr. Jennings’s researches show that no such implication can
be accepted unless we are prepared to cast aside the trammels of
reasonable caution. In the first place, the whole matter of feeding
appears to be referable to simple organic behaviour not necessarily
involving consciousness. The cilia in the mouth-groove and funnel
constantly wave in such a manner as to drive a current of water,
together with any particles which float therein, towards the
interior; and the particles are then engulphed, no matter what
their composition may be. Digestible or indigestible, in they go.
There is no selection of the one or rejection of the other. But, as
we have seen, the Paramecia collect around a bacterial clot and
feed upon it. Surely here there is selection of the nutritious!
Apparently not. They collect in just the same way towards a piece
of blotting-paper, cotton-wool, cloth, sponge, or other fibrous
body, and remain assembled round such an innutritious centre just
as long as round a bacterial clot. There seems to be no choice in
the matter; contact with any substance gives rise, as an organic
response, to the lessening or cessation of the regular movements in
all the cilia except those of the mouth-groove and funnel. As the
Paramecia swim hither and thither, first one, then another, then
more, chance to come in contact with the bacterial clot, the
blotting-paper, or other substance, and since the lashing of the
cilia is then automatically lessened, there they stay; others find
their way to the same spot in the course of their random movements,
and they, too, stay; thus many soon collect.But this does not account for the seemingly social
assemblages of Paramecia where there is no such substance to arrest
their progress. Dr. Jennings attributes this to the fact that a
dilute solution of carbon-dioxide has, what we may call for the
present, an attractive influence. If a bubble of air and a bubble
of carbon dioxide be introduced into the water in which Paramecia
are swimming beneath a cover-glass, the animalcules collect around
the carbonic dioxide, but not around the air bubble. At first they
press up close to the bubble of carbon dioxide, but gradually form
a ring farther and farther from its limiting boundary. This is held
to be due to the fact that it is only the dilute solution of
carbonic acid that has the peculiar “attraction”—a stronger
solution has a different effect. And, as the gas dissolves, the
Paramecia collect in a ring just where the solution is sufficiently
dilute.Now carbon dioxide is a product of the organic waste of
living substance; it is given off by active Paramecia. Where
therefore many are collected together they form a centre of the
production of this substance; and when other Paramecia come, in the
course of their random movements, into such a centre they remain
there and help to swell the numbers in the cluster. If Paramecia be
placed in water to which a distinctly reddish tinge is given by
mixing it with a small quantity of rosol—a substance which is
decolourized by carbon dioxide, and is not injurious to
Paramecia—it will be seen that, where the groups are collected, the
reddish tinge fades and disappears. As the groups expand, and are
less densely packed, the colourless area expands too: and the
limits within which the group is circumscribed are also the limits
of decolourization. Dr. Jennings considers it beyond question that
the assembling of Paramecia is due to the presence in such
assemblages of carbonic acid produced by the animals themselves.
The first beginning of the crowd may be some small fragment of
bacterial clot or other substance.It would seem, then, that Paramecia are attracted by faintly
acid solutions; and here at least there is, it may be urged, an
element of choice. But even here, according to Dr. Jennings, there
is not only no real choice, but not even any real attraction. What
takes place, according to his observations, is briefly as follows.
Suppose a faintly acid drop be inserted beneath the cover-glass.
Paramecia may almost graze its boundary without being in any way
affected by its presence. But in their random movements some, and
eventually many, perhaps most, of the little animals chance to
enter the faintly acid region; but there is no sign of reaction or
response; they swim on across the drop until they reach its further
margin. Here a reaction does take place. Instead of proceeding
onwards, slowly revolving on its long axis, a Paramecium thus
situated jerks backwards by a reversal of all the cilia, at the
same time revolving on its axis in a direction opposite to that in
which it was before turning. But the cilia of the mouth-groove
resume their normal mode of working sooner than the others, and
this causes the Paramecium to turn aside. It then goes ahead until
it again reaches the boundary at another point, when the same
behaviour is seen. The course of such a Paramecium is shown in Fig.
2, IV.If, instead of a faintly acid drop, a little alkaline liquid
be introduced beneath the cover-glass, the Paramecium similarly
jerks backward and turns aside on reaching its outer boundary. The
turning may carry it away from the alkali, as shown in Fig. 2, V.;
but it just as often brings it again towards the drop, especially a
large one. It seems to be a matter of chance which result follows.
But eventually the little creature sails off, since each time it
comes within the influence of the alkaline fluid it jerks back and
turns. It appears, then, that when it is swimming in a normal
solution a faintly acid liquid does not much modify its behaviour,
but an alkaline fluid evokes a reversal of the cilia; and that when
it is a slightly acid solution, not only does stronger acid cause
reversal, but normal fluid produces a similar result. A reaction of
essentially the same kind is in fact called forth by such different
stimuli as chemical substances, water heated above the normal
temperature, or cooled considerably below it, and fluids which
cause changes of internal pressure within the substance of the
cell. Nor does it matter where the stimulus is applied. If it be
applied at the hinder end the infusorian still jerks backward,
though this may drive it into a destructive solution and thus cause
death. There is, however, some evidence of different behaviour in
some infusorians according as the stimulus is here or there. In
other words, the behaviour is to some extent related to the
position of the part stimulated.Furthermore, it may be gathered from Dr. Jennings’s account
that there is nothing to lead us to suppose that such free living
cells show any indication of what may be regarded as the keynote of
intelligent behaviour. They do not profit by experience. They
exhibit organic reactions which may be accompanied by some dim form
of consciousness, but which do not seem to be under the guidance of
such consciousness, if it exist.One of the first lessons which the study of animal behaviour,
in its organic aspect, should impress upon our minds is, that
living cells may react to stimuli in a manner which we perceive to
be subservient to a biological end, and yet react without conscious
purpose—that is to say, automatically. The living cell assimilates
food and absorbs oxygen, it grows and subdivides, it elaborates
secretions, produces a skeletal framework or covering, rids itself
of waste products, responds to stimuli in a definite fashion, moves
hither and thither at random, its functional activities being
stimulated or checked by many influences; and yet this varied life
may give no evidence of a guiding consciousness: if purpose there
be, it lies deeper than its protoplasm, deeper than the dim
sentience which may be present or may be absent—we cannot tell
which.And when the cells are incorporated in the body of one of the
higher animals, instead of each preserving a free and nomad
existence; when they become the multitudinous constituents of an
organic republic with unity of plan and unity of biological end,
then the behaviour of each is limited in range but perfected within
that range, in subservience to the requirements of the more complex
unity. The muscle cell contracts, the gland-cell secretes, the rods
and cones of the retina respond to the waves of light, and all the
normal responses of the special cells go on with such orderly
regularity that the term behaviour seems scarcely applicable to
reactions so stereotyped. But the physiologist and the physician
know well that such uniformity of response is dependent on
uniformity of conditions. A little dose of some drug will
profoundly modify and render abnormal the procedure which was
before so mechanical in its exactitude; and we are thus led to see
how dependent the orderly behaviour really is on the maintenance of
certain surrounding conditions.Moreover, the existence of every cell in the body corporate
is the outcome of a process of division involving a special mode of
behaviour in the nucleus, of which we are only beginning to guess
the meaning and significance, and of which we seek in vain to find
an explanation in mechanical terms. And when we trace these
divisions back to their primary source in the fertilized ovum, we
find changes and evolutions in the nuclear matter of which it can
only be said that the more they are studied the more complex and
varied do they appear.The egg, or ovum, is a single cell produced by the female,
and varying much in size, according to the amount of food-yolk with
which it is supplied. Like other cells, it has a nucleus, and this
undergoes changes which are definitely related to the fertilization
of the ovum, which we describe as the biological end. Such
preparatory changes for a future contingency are especially
characteristic of organic behaviour. There is nothing like it in
the mineral kingdom. The nucleus divides into two parts, one of
which passes out of the ovum and is lost. The nucleus again
divides, and again one part passes out and is lost. Thus only one
quarter of the original amount of nuclear matter remains. Now,
division of the nucleus occurs whenever an animal cell divides; but
in this case (apart from details which would here be out of place)
there is this difference. During the ordinary division of cells
there are found in the nucleus a definite number of curved rods,
and this number is constant for any given species; but in the
nucleus which remains in the ovum after three parts of its
substance are lost, the number of rods has been reduced to half
that which is common to the species. The egg is now ready for
fertilization. A minute active cell, which is produced by the male,
and which also has only half the normal number of rods, enters the
ovum. The two nuclei approach each other, and give rise to the
single nucleus of the fertilized ovum, which thus has the full
number of rods—half of them derived from one parent, half from the
other parent. The sperm cell of the male adds little to the store
of protoplasm in the ovum; but it introduces a minute body, which
seems to initiate subsequent divisions of the cell. The nature of
these divisions may be seen in the accompanying diagrammatic
figure. In A the cell is just preparing to divide. Above the
nucleus is the minute body (centrosome) just spoken of, which has
already divided. In the nucleus the matter of which the rods will
be constituted is net-like. In B this net-work has taken on the new
form of a coiled thread, while the divided body above is associated
with a spindle of delicate fibres. In C the membrane round the
nucleus has disappeared, and the coiled thread has broken up into
curved rods (chromosomes), four of which are shown. The two halves
of the minute body form the centres of radiating stars. In D each
curved rod has split along its length, and the two parts are being
drawn asunder towards the centres of the two stars; the cell itself
is beginning to divide. In E the process is carried a step further,
while in F the cell has completely divided into two: the rods have
disappeared as such, and are replaced by a net-work; a new nuclear
membrane has been formed, and the minute body has again divided
preparatory to the further division of the cell.Fig. 3.—Cell-division.Such, stripped as far as possible of technicalities, are some
of the facts concerning the behaviour of cells and their nuclei
during the process of cell-multiplication. No good purpose would be
subserved by pretending that we fully understand them. The
splitting of the rods does indeed seem an efficient means to the
end of securing a fair division of the nuclear substance, which,
according to many biologists, is the organic bearer of hereditary
qualities in the cells. But that is nearly all that we can say. Is
the process accompanied by some form of sentience? We do not know.
That it is controlled and guided by any consciousness in the cell
is most improbable. But if it be a purely organic and unconscious
process it should at least impress on our minds the fact that such
organic behaviour may reach a high degree of delicacy and
complexity.III.Corporate
BehaviourThe word “corporate” is here applied to the organic behaviour
of cells when they are not independent and free, but are
incorporated in the animal body, and act in relation to each other.
If the behaviour of the individual cell during division impresses
us with the subtle intricacy of organic processes, the behaviour of
the growing cell-republic during the early stages of organic
development must impress us no less forcibly. We place the
fertilized egg of a hen in an incubator, and supply the requisite
conditions of warmth, moisture, and fresh air. Before the egg is
laid cell-division has begun. A small patch of closely similar
cells has formed on the surface of the yolk. Further subdivision is
then arrested until the warmth of incubation quickens again the
patch into life. But when once thus quickened no subsequent
temporary arrest is possible—life will not again lie dormant. If
arrest there be it is that of death. And from that little patch of
cells, which spreads further and further over the yolk, a chick is
developed. Into the intricate technicalities of embryology this is
not the place to enter. But it is a matter of common knowledge
that, whereas we have to-day an egg such as we eat for breakfast,
three weeks hence we shall have a bright active bird, a cunningly
wrought piece of mechanism, and, more than that, a going machine.
During this wonderful process the cellular constituents take on new
forms and perform new functions, all in relationship to each other,
all as part of one organic whole. Here bones are developed to form
a skeletal framework, there muscles are constituted which shall
render orderly movements possible; feathers, beak, and claws take
shape as products of the skin; gut and glands prepare for future
modes of nutrition; heart and blood-vessels undergo many changes,
some reminiscent of bygone and ancestral gill-respiration, some in
relation to the provisional respiration of the embryo by means of a
temporary organ that spreads out beneath the shell, some
preparatory to the future use of the lungs,—some, again, related to
the absorption of food from the yolk, others to subsequent means of
digestion; nerve, brain, and sense-organs differentiate. A going
machine in the egg, the chick is hatched, and forthwith enters on a
wider field of behaviour. Few would think of attributing to the
consciousness of the embryo chick any guiding influence on the
development of its bodily structure, any control over the subtle
changes and dispositions of its constituent cells. But no sooner
does the chick, when it is hatched, begin to show wider modes of
instinctive behaviour, than we invoke conscious intelligence for
their explanation, seemingly forgetful of the fact that there is no
logical ground for affirming that, while the marvellous delicacies
of structure are of unconscious organic origin, the early modes of
instinctive behaviour are due to the guidance of consciousness.
Such modes of behaviour will, however, be considered in another
chapter. Here we have to notice that the unquestionably organic
behaviour of the incorporated republic of cells may attain to a
high degree of complexity, and may serve a distinctly biological
end.Fig. 4.—Wapiti with antlers in velvet.There is, perhaps, no more striking instance of rapid and
vigorous growth than is afforded by the antlers of deer,[3]which are shed and renewed every year. In the early summer,
when growing, they are covered over with a dark hairy skin, and are
said to be “in velvet.” If you lay your hand on the growing antler,
you will feel that it is hot with the nutrient blood that is
coursing beneath it. It is, too, exceedingly sensitive and tender.
An army of tens of thousands of busy living cells is at work
beneath that velvet surface, building the bony antlers, preparing
for the battles of autumn. Each minute cell, working for the
general good, takes up from the nutrient blood the special
materials it requires; elaborates the crude bone-stuff, at first
soft as wax, but ere long to become hard as stone; and then, having
done its work, having added its special morsel to the fabric of the
antler, remains embedded and immured, buried beneath the
bone-products of its successors or descendants. No hive of bees is
busier or more replete with active life than the antler of a stag
as it grows beneath the soft, warm velvet. And thus are built up in
the course of a few weeks those splendid “beams,” with their
“tynes” and “snags,” which, in the case of the wapiti, even in the
confinement of our Zoological Gardens, may reach a weight of
thirty-two pounds, and which, in the freedom of the Rocky
Mountains, may reach such a size that a man may walk, without
stooping, beneath the archway made by setting up upon their points
the shed antlers. When the antler has reached its full size, a
circular ridge makes its appearance at a short distance from the
base. This is the “burr,” which divides the antler into a short
“pedicel” next the skull, and the “beam” with its branches above.
The circulation in the blood-vessels of the beam now begins to
languish, and the velvet dies and peels off, leaving the hard, bony
substance exposed. Then is the time for fighting, when the stags
challenge each other to single combat, while the hinds stand
timidly by. But when the period of battle is over, and the wars and
loves of the year are past, the bone beneath the burr begins to be
eaten away, through the activity of certain large bone-absorbing
cells, and, the base of attachment being thus weakened, the antlers
are shed; the scarred surface skins over and heals, and only the
hair-covered pedicel of the antler is left.Fig. 5.—Wapiti with velvet shredding off.We have no reason to suppose that this corporate cellular
behaviour, involving the nicely adjusted co-operation of so vast an
army of organic units, is under the conscious guidance of the stag.
And yet how orderly the procedure! how admirable the result! Nor is
there an organ or structural part of the stag or any other animal
that does not tell the same tale. This is but one paragraph of the
volume in which is inscribed the varied and wonderful history of
organic behaviour in its corporate aspect. Is it a matter for
wonder that the cause of such phenomena has been regarded as “a
mystery transcending naturalistic conception; as an alien influx
into nature, baffling scientific interpretation”? And yet, though
not surprising, this attitude of mind, in face of organic
phenomena, is illogical, and is due partly to a misconception of
the function of scientific interpretation, partly to influences
arising from the course pursued by the historical development of
scientific knowledge. The function of biological science is to
formulate and to express in generalized terms the related
antecedences and sequences which are observed to occur in animals
and plants. This can already be done with some approach to
precision. But the underlying cause of the observed phenomena does
not fall within the purview of natural science; it involves
metaphysical conceptions. It is no more (and no less) a “mystery”
than all causation in its last resort—as theraison d’êtreof observed phenomena—is
a mystery. Gravitation, chemical affinity, crystalline force,—these
are all “mysteries.”If the mystery of life, lying beneath and behind organic
behaviour, be said to baffle scientific interpretation, this is
because it suggests ultimate problems with which science as such
should not attempt to deal. The final causes of vital phenomena (as
of other phenomena) lie deeper than the probe of science can reach.
But why is this sense of mystery especially evoked in some minds by
the contemplation of organic behaviour, by the study of life?
Partly, no doubt, because the scientific interpretation of organic
processes is but recent, and in many respects incomplete. People
have grown so accustomed to the metaphysical assumptions employed
by physicists and chemists when they speak of the play of
crystalline forces and the selective affinities of atoms, they have
been wont for so long to accept the “mysteries” of crystallization
and of chemical union, that these assumptions have coalesced with
the descriptions and explanations of science; and the joint
products are now, through custom, cheerfully accepted as natural.
Where the phenomena of organic behaviour are in question, this
coalescence has not yet taken place; the metaphysical element is on
the one hand proclaimed as inexplicable by natural science, and on
the other hand denied even by those who talk glibly of physical
forces as the final cause of the phenomena of the inorganic
world.So much reference to the problems which underlie the problems
of science seems necessary. It is here assumed that the phenomena
of organic behaviour are susceptible of scientific discussion and
elucidation. But even assuming that an adequate explanation in
terms of antecedence and sequence shall be thus attained by the
science of the future, this will not then satisfy, any more than
our inadequate explanations now satisfy, those who seek to know the
ultimate meaning and reason of it all: What makes organic matter
behave as we see it behave? what drives the wheels of life, as it
drives the planets in their courses? what impels the egg to go
through its series of developmental changes? what guides the cells
along the divergent course of their life-history? These are
questions the ultimate answers to which lie beyond the sphere of
science—questions which man (who is a metaphysical being) always
does and always will ask, even if he rests content with the answer
of agnosticism; but questions to which natural science never will
be able, and should never so much as attempt, to give an
answer.Enough has now been said to show that organic behaviour is a
thingsui generis, carrying its
own peculiar marks of distinction: and further, that, for science,
this is just part of the constitution of nature, neither more nor
less mysterious than, let us say, crystallization or chemical
combination. But associated and closely interwoven with all that is
distinctively organic there is much which can to some extent be
interpreted in terms of physics and chemistry.The animal[4]has sometimes been likened to a steam-engine, in which the
food is the fuel which enters into combustion with the oxygen taken
in through the lungs. It may be worth while to modify and modernize
this analogy—always remembering, however, that such an analogy must
not be pushed too far.In the ordinary steam-engine the fuel is placed in the
fire-box, to which the oxygen of the air gains access; the heat
produced by the combustion converts the water in the boiler into
steam, which is made to act upon the piston, and thus set the
machinery in motion. But there is another kind of engine, now
extensively used, which works on a different principle. In the
gas-engine the fuel is gaseous, and it can thus be introduced in a
state of intimate mixture with the oxygen with which it is to unite
in combustion. This is a great advantage. The two can unite rapidly
and explosively. In gunpowder the same end is effected by mixing
the carbon and sulphur with nitre, which contains the oxygen
necessary for their explosive combustion. And this is carried still
further in dynamite and gun-cotton, where the elements necessary
for explosive combustion are not merely mechanically mixed, but are
chemically combined in a highly unstable compound.But in the gas-engine, not only are the fuel and the oxygen
thus intimately mixed, but the controlled explosions are caused to
act directly on the piston, and not through the intervention of
water in a boiler. Whereas, therefore, in the steam-engine the
combustion is to some extent external to the working of the
machine, in the gas-engine it is to a large extent internal and
direct.Now, instead of likening the animal as a whole to a
steam-engine, it is more satisfactory to liken each cell to an
automatic gas-engine which manufactures its own explosive. During
the period of repose which intervenes between periods of activity,
its protoplasm is busy in construction, taking from the blood-discs
oxygen, and from the blood-fluid carbonaceous and nitrogenous
materials, and knitting these together into relatively unstable
explosive compounds, which play the part of the mixed air and gas
of the gas-engine. A resting muscle may be likened to a complex and
well-organized battery of gas-engines. On the stimulus supplied
through a nerve-channel a series of co-ordinated explosions takes
place: the gas-engines are set to work; the muscular fibres
contract; the products of the silent explosions are taken up and
hurried away by the blood-stream; and the protoplasm prepares a
fresh supply of explosive material. Long before the invention of
the gas-engine, long before gun-cotton or dynamite were dreamt of,
long before some Chinese or other inventor first mixed the
ingredients of gunpowder, organic nature had utilized the principle
of controlled explosions in the protoplasmic cell, and thus
rendered animal behaviour possible.Certain cells are, however, more delicately explosive than
others. Those, for example, on or near the external surface of the
body—those, that is to say, which constitute the end-organs of the
special senses—contain explosive material which may be fired by a
touch, a sound, an odour, the contact with a sapid fluid or a ray
of light. The effects of the explosions in these delicate cells,
reinforced in certain neighbouring nerve-batteries, are transmitted
down the nerves as waves of subtle chemical or electrolytic change,
and thus reach that wonderful aggregation of organized and
co-ordinated explosive cells, the brain. Here it is again
reinforced and directed (who, at present, can say how?) along fresh
nerve-channels to muscles, or glands, or other organized groups of
explosives. And in the brain, somehow associated with the explosion
of its cells, consciousness, the mind-element, emerges; of which we
need only notice here that it belongs to awholly
different order of beingfrom the physical
activities and products with which we are at present
concerned.We must not press the explosion analogy too far. The
essential thing seems to be that the protoplasm of the cell has the
power of building up complex and unstable chemical compounds, which
are perhaps stored in its spongy substance; and that these unstable
compounds, under the influence of a stimulus (or, possibly,
sometimes spontaneously), break down into simpler and more stable
compounds. In the case of muscle-cells, this latter change is
accompanied by an alteration in length of the fibres, and
consequent movements in the animal, the products of the disruptive
change being useless or harmful, and being, therefore, removed as
soon as possible. But very frequently the products of explosive
activity are made use of. In the case of bone-cells, one of the
products of disruption is of permanent use to the organism, and
constitutes the solid framework of the skeleton. In the case of the
secreting cells in the salivary and other digestive glands, some of
the disruptive products are of temporary value for the preparation
of the food. It is probable that these useful products of
disruption, permanent or temporary, took their origin in waste
products for which natural selection has found a use, and which
have been gradually rendered more and more efficacious in modes of
organic behaviour increasingly complex.In the busy hive of cells which constitutes what we call the
animal body, there is thus ceaseless activity. During periods of
apparent rest the protoplasm is engaged in constructive work,
building up fresh supplies of unstable materials, which, during
periods of apparent activity, break up into simpler and more stable
substances, some of which are useful to the organism, while others
must be got rid of as soon as possible. From another point of view,
the cells during apparent rest are storing up energy to be utilized
by the animal during its periods of activity. The storing up of
available energy may be likened to the winding up of a watch or
clock; it is when an organ is at rest that the cells are winding
themselves up; and thus we have the apparent paradox that the cell
is most active and doing most work when the organ of which it forms
a part is at rest. During the repose of an organ, in fact, the
cells are busily working in preparation for the manifestation of
energetic action that is to follow. Just as the brilliant display
of intellectual activity in a great orator is the result of the
silent work of a lifetime, so is the physical manifestation of
muscular power the result of the silent preparatory work of the
muscle-cells.It may, perhaps, seem strange that the products of cellular
life should be reached by the roundabout process of first producing
unstable compounds, from which are then formed more stable
substances, useful for permanent purposes as in bone, or temporary
purposes as in the digestive fluids. It seems a waste of power to
build up substances unnecessarily complex and stored with an
unnecessarily abundant supply of energy. But only thus could the
organs be enabled to act under the influence of stimuli, and afford
examples of corporate behaviour. They are like charged batteries
ready to discharge under the influence of the slightest organic
touch. In this way, too, is afforded a means by which the organ is
not dependent only upon the products of the immediate activity of
the protoplasm at the time of action, but can utilize the store
laid up during preceding periods of rest.Sufficient has now been said to illustrate the nature of some
of the physical processes which accompany organic behaviour in its
corporate aspect. The fact that should stand out clearly is that
the animal body is stored with large quantities of available energy
resident in highly complex and unstable chemical compounds,
elaborated by the constituent cells. These unstable compounds,
eminently explosive according to our analogy, are built up of
materials derived from two different sources—from the nutritive
matter (containing carbon, hydrogen, and nitrogen) absorbed during
digestion, and from oxygen taken up from the air during
respiration. The cells thus become charged with energy that can be
set free on the application of the appropriate stimulus, which may
be likened to the spark that fires the explosive.Let us note, in conclusion, that it is through the
blood-system, ramifying to all parts of the body, and the
nerve-system, the ramifications of which are not less perfect, that
one of the larger and higher animals is knit together into an
organic whole. The former carries to the cell the raw materials for
the elaboration of its explosive products, and, after the
explosions, carries off the waste products which result therefrom.
The nerve-fibres carry the stimuli by which the explosive is fired,
while the central nervous system organizes, co-ordinates, and
controls the explosions, and initiates the elaboration of the
explosive compounds. Blood and nerves co-operate to render
corporate behaviour possible.IV.—The Behaviour of
PlantsA short parenthetic section on the behaviour of plants may
serve further to illustrate the nature of organic behaviour. We
have seen that Paramecium is apparently attracted by faintly acid
solutions, and have briefly considered Dr. Jennings’s
interpretation of the facts disclosed by careful observation. In
the ferns the female element, or ovum, is contained in a minute
flask-shaped structure (archegonium), in the neck and mouth of
which mucilaginous matter, with a slightly acid reaction, is
developed; and this is said to exercise an attractive influence on
the freely swimming ciliated male elements, or spermatozoids, which
are necessary for fertilization. “Now, it has been shown by
experiment that the spermatozoids of ferns are attracted by certain
chemical substances, and especially by malic acid. If artificial
archegonia are prepared (consisting of tiny capillary glass-tubes)
and filled with mucilage to which a small quantity of this acid has
been added, they are found, when placed in water containing
fern-spermatozoids, to exercise the same attraction upon them which
the real archegonia exercise in nature. The malic acid gradually
diffuses out into the water, and the spermatozoids are influenced
by it, so that they move in the direction in which the substance is
more concentrated,i.e.towards
the tube. Although it cannot be proved that the archegonia
themselves contain malic acid, as they are too small for a
recognizable quantity to be obtained from them, yet there can be
little doubt that the natural archegonia owe their attractive
influence to the same chemical agent which has proved efficacious
in experiment.”[5]In the light of Dr. Jennings’s observations, it is perhaps
not improbable that this so-called attractive influence is similar
to that seen in Paramecium; and that the spermatozoids enter the
organic acid in the course of their random movements, and there
remain. Be that as it may, the male elements collect in the
mucilaginous mass, and pass down the neck of the flask until one
reaches and coalesces with the female element, or ovum, and effects
its fertilization. Here we have organic behaviour unmistakably
directed to a biological end—behaviour which may indeed be
accompanied by some dim form of consciousness, but which is due to
a purely organic reaction. It is scarcely satisfactory to say that
the spermatozoids “possess a certain power of perception, by which
their movements are guided.”[6]If consciousness be present, it is probably merely an
accompaniment of the response, and has no directive influence on
its nature and character.In the higher plants, as in the higher animals, the
differentiation and the orderly marshalling of the cell-progeny
arising from the coalescent male and female elements, afford,
during development, examples of corporate organic behaviour which
can be more readily described than explained, but which not less
clearly subserve definite biological ends, and in many cases, such
as the direction of growth in radicles and roots, the curling of
tendrils, and the reaction to the influence of light and warmth,
are related to and evoked by the environing conditions. More
closely resembling familiar modes of behaviour in animals are such
movements as are seen in the “tentacles” which project from the
upper surface and margin of the Sun-dew leaf. Their knobbed ends
secrete a sticky matter, which glistens in the sun, and to which
small foreign bodies readily adhere. If particles of limestone,
sand, or clay, such as may be blown by the wind, touch and stick to
these knobs, there follows an exudation of acid liquid, but no
marked and continuous change occurs in the position of the
tentacles. But should an insect alight on the leaf, or a small
piece of meat be placed upon the tentacles, not only is there a
discharge of acid juice, but a ferment is also produced, which has
a digestive action on the nitrogenous matter. Slowly the tentacle
curves inwards and downwards, as one’s finger may bend towards the
palm of one’s hand; neighbouring tentacles also turn towards and
incline on to the stimulating substance; then others, further off,
behave in a similar way, until all the tentacles, some two hundred
in number, are inflected and converge upon the nitrogenous
particle. Nay, more: “When two little bits of meat are placed
simultaneously on the right and left halves of the same Sun-dew
leaf, the two hundred tentacles divide into two groups, and each
one of the groups directs its aim to one of the bits of
meat.”[7]Fig. 6.—Sun-dew (Drosera). Leaf (enlarged) with the tentacles on one side inflected
over a bit of meat placed on the disc. (From Darwin’s
“Insectivorous Plants.”)The movements, though slow, are orderly, methodical, and
effective, the secretions of many glands being brought to bear on
just those substances which are capable of digestion and absorption
by the plant. The seemingly concerted action is moreover due to an
organic transmission of impulses from cell to cell—a transmission
accompanied by visible changes in a purple substance contained
within the cells. In the Sun-dew any tentacle may form the
starting-point of the spreading wave of impulse. But in the Venus’s
Fly-trap there are six delicate spines, the slightest touch on any
one of which causes the two halves of the specially modified
leaf-end to fold inwards on the midrib as a hinge. The transmission
of impulse is more rapid, the trap closing in a few seconds; and
electric currents have been observed to accompany the change.
Tooth-like spines at the edge of the trap interlock, and serve to
prevent the escape of small insects, while short-stalked purple
glands secrete an acid digestive juice. Division of labour has been
carried further; and organic behaviour, not less purposive, is
carried out in a manner even more effective.Fig. 7.—Venus’s Fly-trap (Dionæa). Leaf viewed laterally in its
expanded state. (From Darwin’s “Insectivorous Plants.”)In other plants adaptive movements are well known. “Few
phenomena have such a peculiar appearance as the movements which
occur in the sensitive Oxalis when rain comes on. Not only do the
leaflets on which the finest rain-drops fall fold together in a
downward direction, but all the neighbouring ones perform the same
movement, although they have not themselves been shaken by the
impact of the falling drops. The movement is continued to the
common leaf-stalk bearing the numerous leaflets. This also bends
down towards the ground. The rain-drops now slide over the bent
leaf-stalk and down over the depressed leaflets, and not a drop
remains behind on their delicate surfaces.”[8]The waves of impulse are said to be transmitted along
definite lines, and to cause the expulsion of water from certain
cells at the point of insertion of the leaflets or leaf-stalks,
rendering them flaccid.Fig. 8.—Flower ofValisneria.Appealing even more strongly to the popular imagination,
though probably not of deeper biological significance, is the
behaviour of plants in relation to the essential process of
fertilization. Only two examples can here be cited.Valisneria spiralisis an aquatic
plant, with long submerged strap-like leaves, which grows in still
water in Southern Europe. The female flower is enclosed in two
translucent bracts, which form a protective bladder so long as the
flower is beneath the surface of the water; but the flower-stalk
continues to grow until the flower reaches the surface, when it
becomes freely exposed by the splitting of the bracts. There are
three boat-shaped sepals, which act as floats; three quite minute
petals; and three large fringed stigmas, which project over the
abortive petals in the space between the boat-like sepals. The
flower is now ready for fertilization.The male flowers, which are developed on different
individuals from those which produce the female flowers, grow in
bunches beneath an investing bladder. The stalk does not elongate,
so that the bladder never rises far above the bottom, and remains
completely submerged. Here the bladder bursts, and the male
flowers, with short stalks, are detached. Each has three sepals,
which enclose and protect the stamens. The separated flower now
ascends to the surface, the sepals open and form three hollow
boats, by means of which the flower floats freely, while the two
functional stamens project upwards and somewhat obliquely into the
air, exposing the large sticky pollen-cells. Blown hither and
thither by the wind, these little flower-boats “accumulate in the
neighbourhood of fixed bodies, especially in their recesses, where
they rest like ships in harbour. When the little craft happen to
get stranded in the recesses of a female Valisneria flower, they
adhere to the tri-lobed stigma, and some of the pollen-cells are
sure to be left sticking to the fringes on the margins of the
stigmatic surface.”[9]This is a good example of purely organic behaviour admirably
adapted to secure a definite and important biological end. Few will
be likely to contend that it is even accompanied by, still less
under the guidance of, any conscious foresight on the part of the
plant. And the lesson it should teach is that, in the study of
organic behaviour, adaptation to the conditions of existence is not
necessarily the outcome of conscious guidance.It is well known that the orchids exhibit, in their mode of
fertilization, remarkable adaptations by which the visits of
insects are rendered subservient to the needs of the plant. In the
Catasetums, for example, the male flower may be described as
consisting of two parts—a lower part, the cup-like labellum (Fig.
9,l), which constitutes a
landing-stage on which insects may alight; and an upper part, the
column (Fig. 9,c), surrounded
by the upper sepal and petals. In the upper part of the column the
pollen-masses are borne at one end of an elastic pedicel, at the
other end of which is an adhesive disc, and the rod is bent over a
pad so as to be in a state of strain. The disc is retained in
position by a membrane with which two long tubular horns (Figs.
9,h; 10,an) are continuous. These project over
the labellum, where insects alight to gnaw its sweet fleshy walls,
and if they be touched, even very lightly, they convey some
stimulus to the membrane which surrounds and connects the disc with
the adjoining surface, causing it instantly to rupture; and as soon
as this happens, the disc is suddenly set free. The highly elastic
pedicel then flirts the disc out of its chamber with such force
that the whole is ejected, sometimes to a distance of two or three
feet, bringing away with it the two pollen-masses. “The utility of
so forcible an ejection is to drive the soft and viscid cushion of
the disc against the hairy thorax of the large hymenopterous
insects which frequent the flowers. When once attached to an
insect, assuredly no force which the insect could exert would
remove the disc and pedicel, but the caudicles [by which the
pollen-masses are attached] are ruptured without much difficulty,
and thus the balls of pollen might readily be left on the adhesive
stigma of the female flower.”[10]Fig. 9.—Flower ofCatasetum;c, column;h, horns;l, labellum.Here again we have adaptive behaviour of exquisite nicety,
and we have the transmission of an impulse very rapidly along the
cells of the irritable horns, followed by the sudden rupture of a
membrane. Beautiful, however, as is the adaptation, effective as it
is to a definite biological end, the organic behaviour does not
afford any indication of the guidance of consciousness. Among
plants we have many interesting and admirable examples of organic
behaviour; but nowhere so much as a hint of that profiting by
individual experience which is the criterion of the effective
presence of conscious guidance and control.Fig. 10.—Catasetum; C,
diagram of column;a,
anther;an, horn;d, adhesive disc;f, filament of anther;g, ovarium;ped, pedicel; D and E,
pollinium;p, pollen-mass.
(From Darwin’s “Orchids.”)V.—Reflex
ActionIt is sometimes said that the tentacles of the Sun-dew leaf
indicate a primitive kind of reflex action in plants, and that they
afford evidence of discrimination. “It is,” says Romanes, “the
stimulus supplied by continuouspressurethat is so delicately
perceived, while the stimulus supplied byimpactis disregarded.”[11]And, comparing this with what is observed in the Venus’s Fly
Trap, he says: “In these two plants the power of discriminating
between these two kinds of stimuli has been developed to an equally
astonishing extent, but in opposite directions.”[12]It is well, however, to avoid terms which carry with them so
distinctively a conscious implication as “discrimination” and
“perception” do for most of us. Just as the photographer’s film
reacts differently according to the quality of light-rays, violet
or red, which reach it, so do many organic substances react
differently to stimuli of different quality, irrespective of their
intensity. The “discrimination” of plants and of some of the lower
animals is of this kind, and it is better to speak of it simply as
differential reaction. There can then be no chance of its being
confused with conscious choice.Nor should the movements of the Sun-dew tentacles or of those
of the Sea-anemone be termed in strictness reflex action. As
originally employed by Marshall Hall, and, since that time, by
common consent,reflex actioninvolves a differentiated nervous system. There is, first, an
afferent impulse from the point of stimulation passing inwards to a
nerve-centre; secondly, certain little-understood changes within
this centre; and thirdly, an efferent impulse from the centre to
some organ or group of cells which are thus affected. In plants
there is no indication of anything analogous to this specialized
mode of response. The impulse passes directly from the point of
stimulation to the part affected without the intervention of
anything like a nerve-centre. In the sensitive Oxalis the impulse
passes directly to the point of insertion of the leaflet or
leaf-stalk; in Catasetum, from the horn to the retaining membrane;
in the Sun-dew, from the affected tentacle to those in its
neighbourhood. Even in the Sea-anemone, though there is a loosely
diffused nervous system, the passage of the impulse from one part
of the circlet of tentacles to other parts, seems to follow a
direct rather than a reflex course, and there do not appear to be
any specialized centres by which the impulses are received and then
redistributed.In all animals in which well-differentiated nervous systems
are found, in which there are distinct nerve-fibres and
nerve-centres, reflex actions, simple or more complicated, occur.
They form the initial steps leading up to the highest types of
organic behaviour. So long as the nervous arcs—afferent fibres,
nerve-centre, and efferent fibres—remain intact reflex acts may be
carried out with great precision and delicacy, even when the higher
centres, which we believe to be those of conscious guidance and
control, have been destroyed. When, for example, the whole of the
brain of a frog has been extirpated and the animal is hung up by
the lower jaw, if the left side be touched with a drop of acid the
left leg is drawn up and begins to scratch at the irritated spot,
and when this leg is held, the other hind leg is, with seemingly
greater difficulty, brought to bear on the same spot. “This,” says
Sir Michael Foster, “at first sight looks like an intelligent
choice.... But a frog deprived of its brain so that the spinal cord
only is left, makes no spontaneous movements at all. Such an entire
absence of spontaneity is wholly inconsistent with the possession
of intelligence.... We are therefore led to conclude that the
phenomena must be explained in some other way than by being
referred to the working of an intelligence.”[13]But if we concede that intelligence is absent, may there not
at least be some consciousness? Sir Michael Foster’s reply to such
a question goes as far as we have any justification for going, even
when we give free rein to conjecture. “We may distinguish,” he
says, “between an active continuous consciousness, such as we
usually understand by the term, and a passing or momentary
condition, which we may speak of as consciousness, but which is
wholly discontinuous from an antecedent or from a subsequent
similar momentary condition; and indeed we may suppose that the
complete consciousness of ourselves, and the similarly complete
consciousness which we infer to exist in many animals, has been
evolved out of such a rudimentary consciousness. We may, on this
view, suppose that every nervous action of a certain intensity or
character is accompanied by some amount of consciousness which we
may, in a way, compare to the light emitted when a combustion
previously giving rise to invisible heat waxes fiercer. We may thus
infer that when the brainless frog is stirred by some stimulus to a
reflex act, the spinal cord is lit up by a momentary flash of
consciousness coming out of the darkness and dying away into
darkness again; and we may perhaps infer that such a passing
consciousness is the better developed the larger the portion of the
cord involved in the reflex act and the more complex the movement.
But such a momentary flash, even if we admit its existence, is
something very different from consciousness as ordinarily
understood, is far removed from intelligence, and cannot be
appealed to as explaining the ‘choice’ spoken of above.”[14]These sentences indicate with sufficient clearness the
distinction, more than once hinted at in the foregoing pages,
between consciousness as an accompaniment, and consciousness as a
guiding influence. We shall have more to say in this connection in
subsequent chapters. The experiment with the frog shows, at any
rate, that reflex actions, of a distinctly purposive nature, may be
carried out when the centres, which are believed to exercise
conscious control and guidance have been destroyed. It is said that
in man, when, owing to injuries of the spine, the connection
between the brain and the lower part of the spinal cord have been
severed, tickling of the foot causes withdrawal of the limb without
directly affecting the consciousness of the patient. But in all
such cases we are dealing with a maimed creature. The living frog
or man, healthy and intact, is, presumably in the one case,
certainly in the other, conscious of these reflex actions, and can
exercise some amount of guidance and control over them. In man this
is unquestionably the case. But granting that the brain is the
organ of conscious control, granting that it can receive impulses
from and transmit impulses to the reflex centres, no more is here
implied, and no more can be legitimately inferred, than that the
kind of organic behaviour we call “reflex action” is in the higher
animals in touch with the guiding centres. We have no ground for
assuming that in reflex action there is any power of intelligent
guidance independent of that which is exercised by the brain or
analogous organ. In brief, reflex acts, in animals endowed with
intelligence, may be regarded as specialized modes of organic
behaviour; which are in themselves often characterized by much
complexity; which subserve definite biological ends; which are
effected by subordinate centres capable of transmitting impulses
to, and receiving impulses from, the centres of intelligent
guidance; and which, as responses confined to certain organs or
parts of the body, form elements in the wider behaviour of the
animal as a whole.VI.—The Evolution of
Organic BehaviourThe interpretation of organic behaviour in terms of evolution
mainly depends on the answer we give to the question: Are acquired
modes of behaviour inherited? A negative answer to this question is
here provisionally accepted. But the premisses from which this
conclusion is drawn are too technical for discussion in these
pages. It must suffice to state as briefly as possible what this
conclusion amounts to, and to indicate some of the consequences
which follow from its acceptance.The fertilized egg gives origin, as we have seen, to the
multitude of cells which build up the body of one of the higher
animals. There are, on the one hand, muscle-cells, gland-cells,
nerve-cells, and other constituents of the various tissues; and
there are, on the other hand, the reproductive cells—ova or sperms,
as the case may be. Now, every cell in the developed animal is a
direct descendant of the fertilized egg. But of all the varied host
only the reproductive cells take any direct share in the continuity
of the race. Hereditary transmission is therefore restricted to the
germinal substance of these reproductive cells. Trace the ancestry
of any cell in the adult body, say a nerve-cell, and you reach the
fertilized ovum. Trace back the ancestral line yet further, and you
follow a long sequence of reproductive cells, or, at least, of
cells which have undergone but little differentiation; but never
again will you find, in the course of a genealogy of bewildering
length, a nerve-cell. Such a tissue-element is a descendant, but
cannot become an ancestor; it dies without direct heirs.It is universally admitted that the bodily structures are
subject to what is termedmodificationunder the stress of environing circumstances. The muscles may
acquire unusual strength by use and exercise; the nerve-centres may
learn certain tricks of behaviour in the course of individual life;
and other structures may be similarlyaccommodatedto the conditions which
affect them. To such modifications of structure or function in the
organs or parts the termacquiredis primarily applied. The tissues have thus a certain amount
of organic plasticity, through which they are adjusted to a range
of circumstances varying in extent. They are able to acquire new
modes of behaviour. But the cells of which they are composed are
off the line of racial descent. They leave no direct heirs. When
the body dies the modifications of behaviour acquired by its parts
perish with it. Only if in some way they exercise what we may term
a homœopathic influence on the germinal substance can the
accommodation they have learnt be transmitted in inheritance. By
ahomœopathicinfluence is here
meant one that is of such a nature as to communicate to the
germinal substance, the seeds of similar changes of structure or
function. And of the occurrence of any such homœopathic influence
there is no convincing evidence.Logically contrasted with the modifications of the tissues,
dependent on organic plasticity, are thevariationswhich arise from the nature
and constitution of the reproductive cells. How they arise cannot
here be discussed. But they are, it is believed, subject to the
influence of natural selection, which has guided them, throughout
the ages of organic evolution, in the directions they have taken;
disadvantageous variations having been eliminated, and favourable
variations surviving in the struggle for existence. Such modes of
behaviour as are congenital and are due to hereditary transmission
are therefore the outcome of variations which have been selected
generation after generation. And the fit adjustment of this
congenital behaviour to the needs of life is termedadaptation [...]