The question remaining to be asked is--What causes the specialization

of contractile substance?--What causes the growth of colloid masses

which monopolize this contractility, and leave kindred colloids to

monopolize other properties? Has natural selection gradually localized

and increased the primordial muscular substance? or has the frequent

recurrence of irritations and consequent contractions at particular

parts done it? We have, I think, reason to conclude that direct

equilibration rather than indirect equilibration has been chiefly

operative. The reasoning that was used in the case of nerve applies

equally in the case of muscle. A portion of undifferentiated tissue

containing a predominance of the colloid that contracts in changing,

will, during each change, tend to form new molecules of its own type

from the other colloids diffused through it: the tendency of these

entangled colloids to fall into unity with those around them, will

be aided by every shock of isomeric transformation. Hence, repeated

contractions will further the growth of the contracting mass, and

advance its differentiation and integration. If, too, we remember that

the muscular colloid is made to contract by mechanical disturbance, and

that among mechanical disturbances one which will most readily affect

it simultaneously throughout its mass is caused by stretching, we

shall be considerably helped towards understanding how the contractile

tissues are developed. If extension of a muscular colloid previously

at rest, produces in it that molecular disturbance which leads to

isomeric change and decrease of bulk, then there is no difficulty in

explaining the movements of cilia; the formation of a contractile

layer in the vascular system becomes comprehensible; each dilatation

of a blood-vessel caused by a gush of blood, will be followed by a

constriction; the heart will pulsate violently in proportion as it is

violently distended; arteries will develop in power as the stress upon

them becomes greater; and we shall similarly have an explanation of the

increased muscularity of the alimentary canal which is brought about by

increased distension of it.

That the production of contractile tissue in certain localities,

is due to the more frequent excitement in those localities of the

contractility possessed by undifferentiated tissue in general, is a

view harmonizing with traits which the differentiated contractile

tissue exhibits. These are the relations between muscular exercise,

muscular power, and muscular structure; and it is the more needful

for us here to notice them because of certain anomalies they present,

which, at first sight, seem inconsistent with the belief that the

functionally-determined modifications of muscle are inheritable.

Muscles disagree greatly in their tints: all gradations between white

and deep red being observable. Contrasts are visible between the

muscles of different animals, between the muscles of the same animal

at different ages, and between different muscles of the same animal

at the same age. We will glance at the facts under these heads:

noting under each of them the connexion which here chiefly concerns

us--that between the activity of muscle and its depth of colour. The

cold-blooded _Vertebrata_ are, taken as a group, distinguished

from the warmblooded by the whiteness of their flesh; and they are

also distinguished by their comparative inertness. Though a fish

or a reptile can exert considerable force for a short time, it is

not capable of prolonged exertion. Birds and mammals show greater

endurance along with the darker-coloured muscles. If among birds

themselves or mammals themselves we make comparisons, we meet with

kindred contrasts--especially between wild and domestic creatures of

allied kinds. Barn-door fowls are lighter-fleshed than most untamed

gallinaceous birds; and among these last the pheasant, moving about but

little, is lighter-fleshed than the partridge and the grouse which are

more nomadic. The muscles of the sheep are not on the average so dark

as those of the deer; and it is said that the flesh of the wild-boar is

darker than that of the pig. Perhaps, however, the contrast between the

hare and the rabbit affords, among familiar animals, the best example

of the alleged relation: the dark-fleshed hare having no retreat and

making wide excursions, while the white-fleshed rabbit, passing a

great part of its time in its burrow, rarely wanders far from home.

The parallel contrast between young and old animals has a parallel

meaning. Veal is much whiter than beef, and lamb is of lighter colour

than mutton. Though at first sight these facts may not seem to furnish

confirmatory evidence, since lambs in their play appear to expend

more muscular force than their sedate dams; yet the meaning of the

contrast is really as alleged. For in consequence of the law that the

strains which animals have to overcome, increase as the cubes of the

dimensions, while their powers of overcoming them increase only as

the squares (§ 46), the movements of an adult animal cost much more

in muscular effort than do those of a young animal: the result being

that the sheep and the cow exercise their muscles more vigorously in

their quiet movements, than the lamb and the calf in their lively

movements. It may be added as significant, that the domestic animal

in which no very marked darkening of the flesh takes place along with

increasing age, namely the pig, is one which, ordinarily kept in a sty,

leads so quiescent a life that the assigned cause of darkening does

not come into action. But perhaps the most conclusive evidences are

the contrasts which exist between the active and inactive muscles of

the same animal. Between the leg-muscles of fowls and their pectoral

muscles, the difference of colour is familiar; and we know that fowls

exercise their leg-muscles much more than the muscles which move their

wings. Similarly in the turkey, in the guinea fowl, in the pheasant.

And then, adding much to the force of this evidence, we see that in

partridges and grouse, which belong to the same order as our domestic

fowls but use their wings as constantly as their legs, little or

no difference is visible between the colour of these two groups of

muscles. Special contrasts like these do not, however, exhaust the

proofs; for there is a still more significant general contrast. The

muscle of the heart, which is the most active of all muscles, is the

darkest of all muscles.

The connexion of phenomena thus shown in so many ways, implies that

the bulk of a muscle is by no means the sole measure of the quantity

of force it can evolve. It would seem that, other things equal, the

depth of colour varies with the constancy of action; while, other

things equal, the bulk varies with the amount of force that has to be

put forth upon occasion. These of course are approximate relations.

More correctly we may say that the actions of pale muscles are either

relatively feeble though frequent (as in the massive flanks of a fish),

or relatively infrequent though strong (as in the pectoral muscles of

a common fowl); while the actions of dark muscles are both frequent

and strong. Some such differentiation may be anticipated by inference

from the respective physiological requirements. A muscle which has

upon occasion to evolve considerable force, but which has thereafter a

long period of rest during which repair may restore it to efficiency,

requires neither a large reserve of the contractile substance that is

in some way deteriorated by action, nor highly developed appliances for

bringing it nutritive materials and removing effete products. Where,

contrariwise, an exerted muscle which has undergone much molecular

change in evolving much mechanical force, has soon again to evolve

much mechanical force, and so on continually; it is clear that either

the quantity of contractile substance present must be great, or the

apparatus for nutrition and depuration must be very efficient, or both.

Hence we may look for marked unlikenesses of minute structure between

muscles which are markedly contrasted in activity. And we may suspect

that these conspicuous contrasts of colour between active and inactive

muscles, are due to these implied differences of minute structure:

partly differences between the numbers of blood-vessels and partly

differences between the quantities or qualities of sarcous matter.

Here, then, we have a key to the apparent anomaly above hinted at--the

maintenance of bulk by certain muscles which have been rendered

comparatively inactive by changed habits of life. That the pectoral

muscles of those domestic birds which fly but little, have not dwindled

to any great extent, has been thought a fact at variance with the

conclusion that functionally-produced adaptations are inheritable.

It has been argued that if parts which are exercised increase, not

only in the individual but in the race, while parts which become

less active decrease; then a notable difference of size should exist

between the muscles used for flight in birds that fly much, and those

in birds of an allied kind that fly little. But, as we here see, this

is not the true implication. The change in such cases must be chiefly

in vascularity and abundance of contractile substance; and cannot be,

to any great extent, in bulk. For a bird to fly at all, its pectoral

muscles, bones of attachment, and all accompanying appliances, must be

kept up to a certain level of power. If the parts dwindle much, the

creature will be unable to lift itself from the ground. Bearing in mind

that the force which a bird expends to sustain itself in the air during

each successive instant of a short flight is, other things equal,

the same as it expends in each successive instant of a long flight,

we shall see that the muscles employed in the two cases must have

something like equal intensities of contractile power; and that the

structural differences between them must have relation mainly to the

lengths of time during which they can continue to repeat contractions

of like intensity. That is to say, while the power of flight is

retained at all, the muscles and bones cannot greatly dwindle; but the

dwindling, in birds whose flights are short or infrequent or both,

will be in the reserve stock of the substance that is incapacitated by

action, or in the appliances that keep the apparatus in repair, or in

both. Only where, as in the struthious birds, the habit of flight is

lost, can we expect atrophy of all the parts concerned in flight; and

here we find it.

Are such differentiations among the muscles functionally-produced? or

are they produced by the natural selection of variations distinguished

as spontaneous? We have, I think, good grounds for concluding that

they are functionally-produced. We know that in individual men and

animals, the power of sustained action in muscles is rapidly adaptable

to the amount of sustained action required. We know that being “out of

condition,” is usually less shown by the inability to put out a violent

effort than by the inability to continue making violent efforts; and

we know that the result of training for prize-fights and races, is more

shown in the prolongation of energy than in the intensification of

energy. At the same time, experience has taught us that the structural

change which accompanies this functional change, is not so much a

change in the bulk of the muscles as a change in their internal state:

instead of being soft and flabby they become hard. We have inductive

proof, then, that exercise of a muscle causes some interstitial

growth along with the power of more sustained action; and there can

be no doubt that the one is a condition to the other. What is this

interstitial growth? There is reason to suspect that it is in part an

increased deposit of the sarcous substance and in part a development of

blood-vessels. Microscopic observation tends to confirm the conclusions

before drawn, that repetition of contractions furthers the formation

of the matter which contracts, and that greater draughts of blood

determine greater vascularity. And if the contrasts of molecular

structure and the contrasts of vascularity, directly caused in muscles

by contrasts in their activities, are to any degree inheritable;

there results an explanation of those constitutional differences in

the colours and textures of muscles, which accompany constitutional

differences in their degrees of activity.

It may be added that if we are warranted in so ascribing the

differentiations of muscles from one another to direct equilibration,

then we have the more reason for thinking that the differentiation

of muscles in general from other structures is also due to direct

equilibration. That unlikenesses between parts of the contractile

tissues having unlike functions, are caused by the unlikenesses of

their functions, renders it the more probable that the unlikenesses

between contractile tissue and other tissues, have been caused by

analogous unlikenesses.

§ 304. These interpretations, which have already occupied too

large a space, must here be closed. Of course out of phenomena so

multitudinous and varied, it has been impracticable to deal with any

but the most important; and it has been practicable to deal with these

only in a general way. Much, however, as remains to be explained, I

think the possibility of tracing, in so many cases, the actions to

which these internal differentiations may rationally be ascribed,

makes it likely that the remaining internal differentiations are

due to kindred actions. We find evidence that, in more cases than

seemed probable, these actions produce their effects directly on the

individual; and that the unlikenesses are produced by accumulation

of such effects from generation to generation. While for all the

other unlikenesses, we have, as an adequate cause, the indirect

effects wrought by the survival, generation after generation, of the

individuals in which favourable variations have occurred--variations

such as those of which human anatomy furnishes endless instances.

Thus accounting for so much, we may not unreasonably presume that

these co-operative processes of direct and indirect equilibration will

account for what remains.

* * * * *

[NOTE.--After having dismissed this revised chapter as done

with, and sent it to the printer, further thought concerning those

differentiations which produce bone, has reminded me of a fact of

extreme and varied significance named in the first volume. I refer

to the formation of adaptive structures round the ends of dislocated

bones, and to the formation of “false joints.”

These are ontogenetic changes of which phylogeny yields no explanation.

They do not repeat the traits of ancestral organisms, and they cannot

be ascribed to either of the recognized evolutionary factors. If a

humerus be broken across and, failing to set, presently comes to have

its two loose ends so modified as in a measure to simulate the parts of

a normal joint--the ends becoming smooth, covered with periosteum and

supplied with fibrous tissue, and attached by ligaments in such ways

as to allow of restrained movements--it is impossible to think that

natural selection has had anything to do with the power of adjustment

thus shown. No survival of individuals in which adaptations of this

kind, now in one place and now in another, were better and better

effected, could account for acquirement of the ability. Nor can it be

supposed that the ability might result from a functionally-produced

habit; since it is scarcely conceivable that the number of cases in

which individuals profited by it (at first a little and gradually more)

could be such (even did they survive) as to affect the constitution of

the species. Both of the alleged causes of structural modifications

are out of court. It is manifest, too, that the foregoing hypothesis

respecting bone-formation yields us not the slightest help.

But on carefully considering the facts, certain phenomena of profound

meaning may strike us. Here, in a part of the body where no such

tissues ordinarily exist and to which no such structures are ordinarily

appropriate, there arise tissues and structures adapted to the physical

circumstances imposed on that part. Out of what do these abnormal

but appropriate tissues arise? The substances around--osseous,

cartilaginous, membranous--consist of differentiated elements too

far specialized to allow of transformation. These new tissues, then,

must originate from the undifferentiated protoplasm pervading the

part. The units of this protoplasm, subject to the actions proper

to an articulation, begin to assume the appropriate histological

traits--are determined by local stimuli to form tissues ordinarily

associated with such stimuli. What is the inevitable implication? These

units--physiological or constitutional, as we may call them--must

have possessed latent potentialities of falling into these special

arrangements under stress of such conditions. At one point there

arises periosteum and at another ligamentous tissue, while for the

shaping of the ends of the bones--here into a rude hinged form and

there into a rude ball-and-socket form, according to the habitual

movements--there goes on some appropriate deposit of bone. Hence we

must conclude that in the units of protoplasm which have not yet been

organized into special tissues, there resides the ability to take on

one or other type of histological structure according to circumstances;

and, further, that there resides in each of them the still more

marvellous ability to co-operate with kindred units dispersed around

in developing that arrangement of the parts required to constitute a

“false joint.” So that while these units have a general proclivity

towards the structure of the organism as a whole, they have also

proclivities towards structures proper to the local conditions into

which they fall. There is latent in each unit the constitution of the

entire organism and by implication the constitution of every organ; and

each unit while co-operating with the aggregate is ready to take part

in that particular arrangement proper to the position it has fallen

into. If the reader will refer back to §§ 97_d_, 97_e_, in which it is

shown that each member of a human society possesses a combination of

potentialities like these, he will be the better enabled to believe

that this thing _may be_ so while he is unable to conceive how it _is_

so.

And here, indeed, let it be pointed out how completely irrelevant is

the test of conceivableness as applied to these ultimate physiological

actions. For as here, from the un-united ends of the broken bone, there

presently arises a rude joint with fit membranes, ligaments, and even

synovial fluid, though we are absolutely unable to imagine the process

by which the adjacent tissues produce this structure; so there may

be from an organ enlarged by function, such reactive effect upon the

system at large as eventually to influence the reproductive cells,

though we may be absolutely unable to imagine how this can be done.]

CHAPTER IX.

PHYSIOLOGICAL INTEGRATION IN ANIMALS.

§ 305. Physiological differentiation and physiological integration,

are correlatives that vary together. We have but to recollect the

familiar parallel between the division of labour in a society and the

physiological division of labour, to see that as fast as the kinds

of work performed by the component parts of an organism become more

numerous, and as fast as each part becomes more restricted to its

own work, so fast must the parts have their actions combined in such

ways that no one can go on without the rest and the rest cannot go on

without each one.

Here our inquiry must be, how the relationship of these two processes

is established--what causes the integration to advance _pari passu_

with the differentiation. Though it is manifest, _à priori_, that

the mutual dependence of functions must be proportionate to the

specialization of functions; yet it remains to find the mode in which

the increasing co-ordination is determined.

Already, among the Inductions of Biology, this relation between

differentiation and integration has been specified and illustrated

(§ 59). Before dealing with it deductively, a few further examples,

grouped so as to exhibit its several aspects, will be advantageous.

§ 306. If the lowly-organized _Planaria_ has its body broken up and

its gullet detached, this will, for a while, continue to perform

its function when called upon, just as though it were in its place:

a fragment of the creature’s own body placed in the gullet, will

be propelled through it, or swallowed by it. But, as the seeming

strangeness of this fact implies, we find no such independent actions

of analogous parts in the higher animals. Again, a piece cut out of the

disc of a _Medusa_ continues with great persistence repeating those

rhythmical contractions which we see in the disc as a whole; and thus

proves to us that the contractile function in each portion of the disc,

is in great measure independent. But it is not so with the locomotive

organs of more differentiated types. When separated from the rest these

lose their powers of movement. The only member of a vertebrate animal

which continues to act after detachment, is the heart; and the heart

has motor powers complete within itself.

Where there is this small dependence of each part upon the whole,

there is but small dependence of the whole upon each part. The longer

time which it takes for the arrest of a function to produce death in

a less-differentiated animal than in a more-differentiated animal,

may be illustrated by the case of respiration. Suffocation in a man

speedily causes resistance to the passage of the blood through the

capillaries, followed by congestion and stoppage of the heart: great

disturbance throughout the system results in a few seconds, and in

a minute or two all the functions cease. But in a frog, with its

undeveloped respiratory organ, and a skin through which a considerable

aëration of the blood is carried on, breathing may be suspended for

a long time without injury. Doubtless this difference is proximately

due to the greater functional activity in the one case than in the

other, and the more pressing need for discharging the produced carbon

dioxide; but the greater functional activity being itself made possible

by the higher specialization of functions, this remains the primary

cause of the greater dependence of the other functions on respiration,

where the respiratory apparatus has become highly specialized. Here

indeed, we see the relation under another aspect. This more rapid

rhythm of the functions which increased heterogeneity of structure

makes possible, is itself a means of integrating the functions. Watch,

when it is running down, a complicated machine of which the parts are

not accurately adjusted, or are so worn as to be somewhat loose. There

will be observed certain irregularities of movement just before it

comes to rest--certain of the parts which stop first, are again made to

move a little by the continued movement of the rest, and then become

themselves, in turn, the causes of renewed motion in other parts which

have ceased to move. That is to say, while the connected rhythmical

changes of the machine are quick, their actions and reactions on one

another are regular--all the motions are well integrated; but as the

velocity diminishes irregularities arise--the motions become somewhat

disintegrated. Similarly with organic functions: increase of their

rapidity involves increase of a joint momentum which controls each

and co-ordinates all. Thus if we compare a snake with a mammal, we

see that its functions are not tied together so closely. The mammal,

and especially the superior mammal, requires food with considerable

regularity; keeps up a respiration which varies within but moderate

limits; and has periods of activity and rest that alternate evenly and

frequently. But the snake, taking food at long intervals, may have

these intervals greatly extended without fatal results; its dormant and

its active states recur less uniformly; and its rate of respiration

varies within much wider limits--now being scarcely perceptible and

now, as you may prove by exciting it, becoming conspicuous. So that

here, where the rhythms are very slow, they are individually less

regular, and are united into a less regular compound rhythm--are less

integrated.

Perhaps the clearest general idea of the co-ordination of functions

that accompanies their specialization, is obtained by observing the

slowness with which a little-differentiated animal responds to a

stimulus applied to one of its parts, and the rapidity with which

such a local stimulus is responded to by a more-differentiated animal.

A sea-anemone and a fly will serve for the comparison. A tentacle of

a sea-anemone, when touched, slowly contracts; and if the touch has

been rude, the contraction presently extends to the other tentacles

and eventually to the entire body: the stimulus to movement is

gradually diffused throughout the organism. But if you touch a fly,

or rather if you come near enough to threaten a touch, the entire

apparatus of flight is instantly brought into combined action. Whence

arises this contrast? The one creature has but faintly specialized

contractile organs, and fibres for conveying impressions. The other

has definite muscles and nerves and a co-ordinating centre. The parts

of the little-differentiated sea-anemone have their functions so

feebly co-ordinated, that one may be strongly affected for some time

before any effect is felt by another at a distance from it; but in the

much-differentiated fly, various remote parts instantly have changes

propagated to them from the affected part, and by their united actions

thus set up, the whole organism adjusts itself so as to avoid the

danger.

These few added illustrations will make the nature of this general

relation sufficiently clear. Let us now pass to the interpretation of

it.

§ 307. If a _Hydra_ is cut in two, the nutritive liquids diffused

through its substance cannot escape rapidly, since there are no

open channels for them; and hence the conditions of the parts at

a distance from the cut is but little affected. But where, as in

the more-differentiated animals, the nutritive liquid is contained

in vessels which have continuous communications, cutting the body

in two, or cutting off any considerable portion of it, is followed

by escape of the liquid from these vessels to a large extent; and

this affects the nutrition and efficiency of organs remote from the

place of injury. Then where, as in further-developed creatures,

there exists an apparatus for propelling the blood through these

ramifying channels, injury of a single one will cause a loss of blood

that quickly prostrates the entire organism. Hence the rise of a

completely-differentiated vascular system, is the rise of a system

which integrates all members of the body, by making each dependent on

the integrity of the vascular system, and therefore on the integrity

of each member through which it ramifies. In another mode, too, the

establishment of a distributing apparatus produces a physiological

union that is great in proportion as this distributing apparatus is

efficient. As fast as it assumes a function unlike the rest, each part

of an animal modifies the blood in a way more or less unlike the rest,

both by the materials it abstracts and by the products it adds; and

hence the more differentiated the vascular system becomes, the more

does it integrate all parts by making each of them feel the qualitative

modification of the blood which every other has produced. This is

simply and conspicuously exemplified by the lungs. In the absence of a

vascular system, or in the absence of one that is well marked off from

the imbedding tissues, the nutritive plasma or the crude blood, gets

what small aëration it can, only by coming near the creature’s outer

surface, or those inner surfaces which are bathed by water. But where

there have been formed definite channels branching throughout the body,

and particularly where there exist specialized organs for pumping the

blood through these channels, it manifestly becomes possible for the

aëration to be carried on in one part peculiarly modified to further

it, while all other parts have the aërated blood brought to them. And

how greatly the differentiation of the vascular system thus becomes a

means of integrating the various organs, is shown by the fatal result

that follows when the current of aërated blood is interrupted.

Here, indeed, it becomes obvious both that certain physiological

differentiations make possible certain physiological integrations;

and that, conversely, these integrations make possible other

differentiations. Besides the waste products which escape through the

lungs, there are waste products which escape through the skin, the

kidneys, the liver. The blood has separated from it in each of these

structures, the particular product which this structure has become

adapted to separate; leaving the other products to be separated by

the other adapted structures. How have these special adaptations been

made possible? By union of the organs as recipients of one circulating

mass of blood. While there is no efficient apparatus for transfer of

materials through the body, the waste products of each part have to

make their escape locally; and the local channels of escape must be

competent to take off indifferently all the waste products. But it

becomes practicable and advantageous for the differently-localized

excreting structures to become fitted to separate different waste

products, as soon as the common circulation through them grows so

efficient that the product left unexcreted by one is quickly carried to

another better fitted to excrete it. So that the integration of them

through a common vascular system, is the condition under which only

they can become differentiated. Perhaps the clearest idea of the way in

which differentiation leads to integration, and how, again, increased

integration makes possible still further differentiation, will be

obtained by contemplating the analogous dependence in the social

organism. While it has no roads, a country cannot have its industries

much specialized: each locality must produce, as best it can, the

various commodities it consumes, so long as it has no facilities for

barter with other localities. But the localities being unlike in their

natural fitnesses for the various industries, there tends ever to

arise some exchange of the commodities they can respectively produce

with least labour. This exchange leads to the formation of channels

of communication. The currents of commodities once set up, make

their foot-paths and horse-tracks more permeable; and as fast as the

resistance to exchange becomes less, the currents of commodities become

greater. Each locality takes more of the products of adjacent ones,

and each locality devotes itself more to the particular industry for

which it is naturally best fitted: the functional integration makes

possible a further functional differentiation. This further functional

differentiation reacts. The greater demand for the special product of

each locality, excites improvements in production--leads to the use of

methods which both cheapen and perfect the commodity. Hence results a

still more active exchange; a still clearer opening of the channels of

communication; a still closer mutual dependence. Yet another influence

comes into play. As fast as the intercourse, at first only between

neighbouring localities, makes for itself better roads--as fast as

rivers are bridged and marshes made easily passable, the resistance to

distribution becomes so far diminished, that the things grown or made

in each district can be profitably carried to a greater distance; and

as the economical integration is thus extended over a wider area, the

economical differentiation is again increased; since each district,

having a larger market for its commodity, is led to devote itself more

exclusively to producing this commodity. These actions and reactions

continue until the various localities, becoming greatly developed

and highly specialized in their industries, are at the same time

functionally integrated by a network of roads, and finally railways,

along which rapidly circulate the currents severally sent out and

received by the localities. And it will be manifest that in individual

organisms a like correlative progress must have been caused in an

analogous way.

§ 308. Another and higher form of physiological integration in animals,

is that which the nervous system effects. Each part as it becomes

specialized, begins to act upon the rest not only indirectly through

the matters it takes from and adds to the blood, but also directly

through the molecular disturbances it sets up and diffuses. Whether

nerves themselves are differentiated by the molecular disturbances

thus propagated in certain directions, or whether they are otherwise

differentiated, it must equally happen that as fast as they become

channels along which molecular disturbances travel, the parts they

connect become physiologically integrated, in so far that a change

in one initiates a change in the other. We may dimly perceive that

if portions of what was originally a uniform mass having a common

function, undertake subdivisions of the function, the molecular changes

going on in them will be in some way complementary to one another:

that peculiar form of molecular motion which the one has lost in

becoming specialized, the other has gained in becoming specialized.

And if the molecular motion that was common to the two portions while

they were undifferentiated, becomes divided into two complementary

kinds of molecular motion; then between these portions there will be

a contrast of molecular motions such that whatever is _plus_ in the

one will be _minus_ in the other; and hence there will be a special

tendency towards a restoration of the molecular equilibrium between

the two: the molecular motion continually propagated away from either

will have its line of least resistance in the direction of the

other. If, as argued in the last chapter, repeated restorations of

molecular equilibrium, always following the line of least resistance,

tend ever to make it a line of diminished resistance; then, in

proportion as any parts become more physiologically integrated by

the establishment of this channel for the easy transmission of

molecular motion between them, they may become more physiologically

differentiated. The contrast between their molecular motions leads to

the line of discharge; the line of discharge, once formed, permits a

greater contrast of their molecular motions to arise; thereupon the

quantities of molecular motion transferred to restore equilibrium,

being increased, the channel of transfer is made more permeable; and

its further permeability, so caused, renders possible a still more

marked unlikeness of action between the parts. Thus the differentiation

and the integration progress hand in hand as before. How the same

principle holds throughout the higher stages of nervous development,

can be seen only still more vaguely. Nevertheless, it is comprehensible

that as functions become further divided, there will arise the

need for sub-connexions along which there may take place secondary

equilibrations subordinate to the main ones. It is manifest, too, that

whereas the differentiation of functions proceeds, not necessarily by

division into two, but often by division into several, and usually in

such ways as not to leave any two functions that are just complementary

to one another, the restorations of equilibrium cannot be so simple

as above supposed. And especially when we bear in mind that many

differentiated functions, as those of the senses, cannot be held

complementary to any other functions in particular; it becomes manifest

that the equilibrations that have to be made in an organism of much

heterogeneity, are extremely complex, and do not take place between

each organ and some other, but between each organ and all the others.

The peculiarity of the molecular motion propagated from each organ,

has to be neutralized by some counter-peculiarity in the average of

the molecular motions with which it is brought into relation. All the

variously-modified molecular motions from the various parts, must have

their pluses and minuses mutually cancelled: if not locally, then at

some centre to which each unbalanced motion travels until it meets with

some opposite unbalanced motion to destroy it. Still, involved as these

actions must become, it is possible to see how the general principle

illustrated by the simple case above supposed, will continue to hold.

For always the molecular motion proceeding from any one differentiated

part, will travel most readily towards that place where a molecular

motion most complementary to it in kind exists--no matter whether this

complementary molecular motion be that proceeding from any one other

organ, or the _resultant_ of the molecular motions proceeding from

many other organs. So that the tendency will be for each channel of

communication or nerve, to unite itself with some centre or ganglion,

where it comes into relation with other nerves. And if there be any

parts of its peculiar molecular motion uncancelled by the molecular

motions it meets at this centre; or if, as will probably happen, the

average molecular motion which it there unites to produce, differs from

the average molecular motion elsewhere; then, as before, there will

arise a discharge along another channel or nerve to another centre

or ganglion, where the residuary difference may be cancelled by the

differences it meets; or whence it may be still further propagated till

it is so cancelled. Thus there will be a tendency to a general nervous

integration keeping pace with the differentiation.

Of course this must be taken as nothing more than the indication of

initial tendencies--not as an hypothesis sufficient to account for all

the facts. It leaves out of sight the origin and functions of ganglia,

considered as something more than nerve-junctions. Were there only

these lines of easy transmission of molecular disturbance, a change

set up in one organ could never do more than produce its equivalent of

change in some other or others; and there could be none of that large

amount of motion initiated by a small sensation, which we habitually

see. The facts show, unmistakably, that the slight disturbance

communicated to a ganglion, causes an overthrow of that highly-unstable

nervous matter contained in it, and a discharge from it of the

greatly-increased quantity of molecular motion so generated. This,

however, is beyond our immediate topic. All we have here to note is the

interdependence and unification of functions that naturally follow the

differentiation of them.

§ 309. Something might be added concerning the further class of

integrations by which organisms are constituted mechanically-coherent

wholes. Carrying further certain of the arguments contained in the

last chapter, it might be not unreasonably inferred that the binding

together of parts by bones, muscles, and ligaments, is a secondary

result of those same actions by which bones, muscles, and ligaments

are specialized. But adequate treatment of this division of the subject

is at present scarcely possible.

What little of fact and inference has been above set down, will,

however, serve to make comprehensible the general truths respecting

which, in their main outlines, there can be no question. Beginning with

the feebly-differentiated sponge, of which the integration is also so

feeble that cutting off a piece interferes in no appreciable degree

with the activity and growth of the rest, it is undeniable that the

advance is through stages in which the multiplication of unlike parts

having unlike actions, is accompanied by an increasing interdependence

of the parts and their actions; until we come to structures like our

own, in which a slight change initiated in one part will instantly and

powerfully affect all other parts--will convulse an immense number of

muscles, send a wave of contraction through all the blood-vessels,

awaken a crowd of ideas with an accompanying gush of emotions, affect

the action of the lungs, of the stomach, and of all the secreting

organs. And while it is a manifest necessity that along with this

subdivision of functions which the higher organisms show us, there

must be this close co-ordination of them, the foregoing paragraphs

suggest how this necessary correlation is brought about. For a great

part of the physiological union that accompanies the physiological

specialization, there appears to be a sufficient cause in the process

of direct equilibration; and indirect equilibration may be fairly

presumed a sufficient cause for that which remains.

CHAPTER X.

SUMMARY OF PHYSIOLOGICAL DEVELOPMENT.

§ 310. Intercourse between each part and the particular conditions

to which it is exposed, either habitually in the individual

or occasionally in the race, thus appears to be the origin of

physiological development; as we found it to be the origin of

morphological development. The unlikenesses of form that arise among

members of an aggregate that were originally alike, we traced to

unlikenesses in the incident forces. And in the foregoing chapters we

have traced to unlikenesses in the incident forces, those unlikenesses

of minute structure and chemical composition that simultaneously arise

among the parts.

In summing up the special truths illustrative of this general truth,

it will be proper here to contemplate more especially their dependence

on first principles. Dealing with biological phenomena as phenomena of

evolution, we have to interpret not only the increasing morphological

heterogeneity of organisms, but also their increasing physiological

heterogeneity, in terms of the re-distribution of matter and motion.

While we make our rapid re-survey of the facts, let us then more

particularly observe how they are subordinate to the universal course

of this re-distribution.

§ 311. The instability of the homogeneous, or, strictly speaking, the

inevitable lapse of the more homogeneous into the less homogeneous,

which we before saw endlessly exemplified by the morphological

differentiations of the parts of organisms, we have here seen

afresh exemplified in ways also countless, by the physiological

differentiations of their parts. And in the one case as in the other,

this change from uniformity to multiformity in organic aggregates, is

caused, as it is in all inorganic aggregates, by the necessary exposure

of their component parts to actions unlike in kind or quantity or

both. General proof of this is furnished by the order in which the

differences appear. If parts are rendered physiologically heterogeneous

by the heterogeneity of the incident forces, then the earliest

contrasts should be between parts that are the most strongly contrasted

in their relations to incident forces; the next earliest contrasts

should occur where there are the next strongest contrasts in these

relations; and so on. It turns out that they do so.

Everywhere the differentiation of outside from inside comes first.

In the simplest plants the unlikeness of the cell-wall to the

cell-contents is the conspicuous trait of structure. The contrasts

seen in the simplest animals are of the same kind: the film that

covers a Rhizopod and the more indurated coat of an Infusorian, are

more unlike the contained sarcode than the other parts of this are

from one another; and the tendency during the life of the animal is

for the unlikeness to become greater. What is true of _Protophyta_ and

_Protozoa_, is true of the germs of all organisms up to the highest:

the differentiation of outer from inner is the first step. When the

protoplasm of an _Alga_-cell has broken up into the clusters of

granules which are eventually to become spores, each of these quickly

acquires a membranous coating, constituting an unlikeness between

surface and centre. Similarly with the ovule of every higher plant:

the mass of cells forming it, early exhibits an outside layer of cells

distinguished from the cells within. With animal-germs it is the

same. Be it in a ciliated gemmule, be it in the unfertilized ova of

_Aphides_ and of the _Cecidomyia_, or be it in true ova, the primary

differentiation conforms to the relations of exterior and interior.

If we turn to adult organisms, vegetal or animal, we see that whether

they do or do not display other contrasts of parts, they always display

this contrast. Though otherwise almost homogeneous, such _Fungi_ as the

puff-ball, or, among _Algæ_, all which have a thallus of any thickness,

present marked differences between those of their cells which are in

immediate contact with the environment and those which are not. Such

differences they present in common with every higher plant; which,

here in the shape of bark and there in the shape of cuticle, has an

envelope inclosing it even up to its petals and stamens. In like manner

among animals, there is always either a true skin or an outer coat

analogous to one. Wherever aggregates of the first order have united

into aggregates of the second and third orders--wherever they have

become the morphological units of such higher aggregates--the outermost

of them have grown unlike those lying within. Even the Sponge is not

without a layer that may by analogy be called dermal.

This lapse of the relatively homogeneous into the relatively

heterogeneous, first showing itself, as on the hypothesis of evolution

it must do, by the rise of an unlikeness between outside and inside,

goes on next to show itself, as we infer that it must do, by the

establishment of secondary contrasts among the outer parts answering to

secondary contrasts among the forces falling on them. So long as the

whole surface of a plant remains similarly related to the environment,

as in a _Protococcus_, it remains uniform; but when there come to be

an attached surface and a free surface, these, being subject to unlike

actions, are rendered unlike. This is visible even in a unicellular

_Alga_ when it becomes fixed; it is shown in the distinction between

the under and upper parts of ordinary _Fungi_; and we see it in the

universal difference between the imbedded ends and the exposed ends of

the higher plants. And then among the less marked contrasts of surface

answering to the less marked contrasts in the incident forces, come

those between the upper and under sides of leaves; which, as we have

seen, vary in degree as the contrasts of forces vary in degree, and

disappear where these contrasts disappear. Equally clear proof is

furnished by animals, that the original uniformity of surface lapses

into multiformity, in proportion as the actions of the environment

upon the surface become multiform. In a Worm, burrowing through damp

soil which acts equally on all its sides, or in a _Tænia_, uniformly

bathed by the contents of the intestine it inhabits, the parts of the

integument do not appreciably differ from one another; but in creatures

not surrounded by the same agencies, as those that crawl and those

that have their bodies partially inclosed, there are unlikenesses of

integument corresponding to unlikenesses of the conditions. A snail’s

foot has an under surface not uniform with the exposed surface of

its body, and this again is not uniform with the protected surface.

Among articulate animals there is usually a distinction between the

ventral and the dorsal aspects; and in those of the _Arthropoda_ which

subject their anterior and posterior ends to different environing

agencies, as do the ant-lion and the hermit-crab, these become

superficially differentiated. Analogous general contrasts occur among

the _Vertebrata_. Fishes, though their outsides are uniformly bathed

by water, have their backs more exposed to light than their bellies,

and the two are commonly distinct in colour. When it is not the back

and belly which are thus dissimilarly conditioned, but the sides, as in

the _Pleuronectidæ_, then it is the sides which become contrasted; and

there may be significance in the fact that those abnormal individuals

of this order which revert to the ancestral undistorted type, and

swim vertically, have the two sides alike. In such higher vertebrates

as reptiles, we see repeated this differentiation of the upper and

under surfaces: especially in those of them which, like snakes, expose

these surfaces to the most diverse actions. Even in birds and mammals

which usually, by raising the under surface considerably above the

ground, greatly diminish the contrast between its conditions and the

conditions to which the upper surface is subject, there still remains

some unlikeness of clothing answering to the remaining unlikeness

between the conditions. Thus, without by any means saying that all such

differentiations are directly caused by differences in the actions of

incident forces, which, as before shown (§ 294), they cannot be, it is

clear that many of them are so caused. It is clear that parts of the

surface exposed to very unlike environing agencies, become very unlike;

and this is all that needs to be shown.

Complex as are the transformations of the inner parts of organisms

from the relatively homogeneous into the relatively heterogeneous, we

still see among them a conformity to the same general order. In both

plants and animals the earlier internal differentiations answer to the

stronger contrasts of conditions. Plants, absorbing all their nutriment

through their outer surfaces, are internally modified mainly by the

transfer of materials and by mechanical stress. Such of them as do not

raise their fronds above the surface, have their inner tissues subject

to no marked contrasts save those caused by currents of sap; and the

lines of lengthened and otherwise changed cells which are formed where

these currents run, and are most conspicuous where these currents must

obviously be the strongest, are the only decided differentiations of

the interior. But where, as in the higher Cryptogams and in Phænogams,

the leaves are upheld, and the supporting stem is transversely bent by

the wind, the inner tissues, subject to different amounts of mechanical

strain, differentiate accordingly: the deposit of dense substance

commences in that region where the sap-containing cells and canals

suffer the greatest intermittent compressions. Animals, or at least

such of them as take food into their interiors, are subject to forces

of another class tending to destroy their original homogeneity. Food

is a foreign substance which acts on the interior as an environing

object which touches it acts on the exterior--is literally a portion

of the environment which, when swallowed, becomes a cause of internal

differentiations as the rest of the environment continues a cause

of external differentiations. How essentially parallel are the two

sets of actions and reactions, we have seen implied by the primordial

identity of the endoderm and ectoderm in simple animals, and of the

skin and mucous membrane in complex animals (§§ 288, 289). Here

we have further to observe that as food is the original source of

internal differentiations, these may be expected to show themselves

first where the influence of the food is greatest; and to appear

later in proportion as the parts are more removed from the influence

of the food. They do this. In animals of low type, the coats of the

alimentary cavity or canal are more differentiated than the tissue

which lies between the alimentary canal and the wall of the body. This

tissue in the higher _Cœlenterata_, is a feebly-organized parenchyma

traversed by canals lined with simple ciliated cells; and in the lower

_Mollusca_ the structures bounding the peri-visceral cavity and its

ramifying sinuses, are similarly imperfect. Further, it is observable

that the differentiation of this peri-visceral sac and its sinuses into

a vascular system, proceeds centrifugally from the region where the

absorbed nutriment enters the mass of circulating liquid, and where

this liquid is qualitatively more unlike the tissues than it is at the

remoter parts of the body.

Physiological development, then, is initiated by that instability

of the homogeneous which we have seen to be everywhere a cause of

evolution (_First Principles_, §§ 149–155). That the passage

from comparative uniformity of composition and minute structure to

comparative multiformity, is set up in organic aggregates, as in all

other aggregates, by the necessary unlikenesses of the actions to which

the parts are subject, is shown by the universal rise of the primary

differentiation into the parts that are universally most contrasted in

their circumstances, and by the rise of secondary differentiations

obviously related in their order to secondary contrasts of conditions.

§ 312. How physiological development has all along been aided by the

multiplication of effects--how each differentiation has ever tended to

become the parent of new differentiations, we have had, incidentally,

various illustrations. Let us here review the working of this cause.

Among plants we see it in the production of progressively-multiplying

heterogeneities of tissue by progressive increase of bulk. The

integration of fronds into axes and of axes into groups of axes,

sets up unlikenesses of action among the integrated units, followed

by unlikenesses of minute structure. Each gust transversely strains

the various parts of the stem in various degrees, and longitudinally

strains in various degrees the roots; and while there is inequality of

stress at every place in stem and branch, so, at every place in stem

and branch, the outer layers and the successively inner layers are

severally extended and compressed to unequal amounts, and have unequal

modifications wrought in them. Let the tree add to its periphery

another generation of the units composing it, and immediately the

mechanical strains on the supporting parts are all changed in different

degrees, initiating new differences internally. Externally, too, new

differences are initiated. Shaded by the leaf-bearing outer stratum

of shoots, the inner structures cease to bear leaves, or to put out

shoots which bear leaves; and instead of that green covering which

they originally had, become covered with bark of increasing thickness.

Manifestly, then, the larger integration of units that are originally

simple and uniform, entails physiological changes of various orders,

varying in their degrees at all parts of the aggregate. Each branch

which, favourably circumstanced, flourishes more than its neighbours,

becomes a cause of physiological differentiations, not only in its

neighbours from which it abstracts sap and presently turns from

leaf-bearers into fruit-bearers, but also in the remoter parts.

That among animals physiological development is furthered by the

multiplication of effects, we have lately seen proved by the many

changes in other organs, which the growth or modification of each

excreting and secreting organ initiates. By the abstracted as well as

by the added materials, it alters the quality of the blood passing

through all members of the body; or by the liquid it pours into the

alimentary canal, it acts on the food, and through it on the blood,

and through it on the system as a whole: an additional differentiation

in one part thus setting up additional differentiations in many other

parts; from each of which, again, secondary differentiating forces

reverberate through the organism. Or, to take an influence of another

order, we have seen how the modified mechanical action of any member

not only modifies that member, but becomes, by its reactions, a cause

of secondary modifications--how, for example, the burrowing habits

of the common mole, leading to an almost exclusive use of the fore

limbs, have entailed a dwindling of the hind limbs, and a concomitant

dwindling of the pelvis, which, becoming too small for the passage of

the young, has initiated still more anomalous modifications.

So that throughout physiological development, as in evolution at large,

the multiplication of effects has been a factor constantly at work, and

working more actively as the development has advanced. The secondary

changes wrought by each primary change, have necessarily become more

numerous in proportion as organisms have become more complex. And

every increased multiplication of effects, further differentiating the

organism and, by consequence, further integrating it, has prepared the

way for still higher differentiations and integrations similarly caused.

§ 313. The general truth next to be resumed, is that these processes

have for their limit a state of equilibrium--proximately a moving

equilibrium and ultimately a complete equilibrium. The changes we have

contemplated are but the concomitants of a progressing equilibration.

In every aggregate which we call living, as well as in all other

aggregates, the instability of the homogeneous is but another name

for the absence of balance between the incident forces and the forces

which the aggregate opposes to them; and the passage into heterogeneity

is the passage towards a state of balance. And to say that in every

aggregate, organic or other, there goes on a multiplication of effects,

is but to say that one part which has a fresh force impressed on it,

must go on changing and communicating secondary changes, until the

whole of the impressed force has been used up in generating equivalent

reactive forces.

The principle that whatever new action an organism is subject to, must

either overthrow the moving equilibrium of its functions and cause the

sudden equilibration called death, or else must progressively alter the

organic rhythms until, by the establishment of a new reaction balancing

the new action a new moving equilibrium is produced, applies as much

to each member of an organism as to the organism in its totality. Any

force falling on any part not adapted to bear it, must either cause

local destruction of tissue, or must, without destroying the tissue,

continue to change it until it can change it no further; that is--until

the modified reaction of the part has become equal to the modified

action. Whatever the nature of the force this must happen. If it is a

mechanical force, then the immediate effect is some distortion of the

part--a distortion having for its limit that attitude in which the

resistance of the structures to further change of position, balances

the force tending to produce the further change; and the ultimate

effect, supposing the force to be continuous or recurrent, is such a

permanent alteration of form, or alteration of structure, or both, as

establishes a permanent balance. If the force is physico-chemical, or

chemical, the general result is still the same: the component molecules

of the tissue must have their molecular arrangements changed, and the

change in their molecular arrangements must go on until their molecular

motions are so re-adjusted as to equilibrate the molecular motions of

the new physico-chemical or chemical agent. In other words, the organic

matter composing the part, if it continues to be organic matter at all,

must assume that molecular composition which enables it to bear, or as

we say adapts it to, the incident forces.

Nor is it less certain that throughout the organism as a whole,

equilibration is alike the proximate limit of the changes wrought by

each action, as well as the ultimate limit of the changes wrought by

any recurrent actions or continuous action. The movements every instant

going on, are movements towards a new state of equilibrium. Raising

a limb causes a simultaneous shifting of the centre of gravity, and

such altered tensions and pressures throughout the body as re-adjust

the disturbed balance. Passage of liquid into or out of a tissue,

implies some excess of force in one direction there at work; and ceases

only when the force so diminishes or the counter-forces so increase

that the excess disappears. A nervous discharge is reflected and

re-reflected from part to part, until it has all been used up in the

re-arrangements produced--equilibrated by the reactions called out. And

what is thus obviously true of every normal change, is equally true of

every abnormal change--every disturbance of the established rhythm of

the functions. If such disturbance is a single one, the perturbations

set up by it, reverberating throughout the system, leave its moving

equilibrium slightly altered. If the disturbance is repeated or

persistent, its successive effects accumulate until they have produced

a new moving equilibrium adjusted to the new force.

Each re-balancing of actions, having for its necessary concomitant a

modification of tissues, it is an obvious corollary that organisms

subjected to successive changes of conditions, must undergo successive

differentiations and re-differentiations. Direct equilibration in

organisms, with all its accompanying structural alterations, is as

certain as is that universal progress towards equilibrium of which

it forms part. And just as certain is that indirect equilibration

in organisms to which the remaining large class of differentiations

is due. The development of favourable variations by the killing of

individuals in which they do not occur or are least marked, is, as

before, a balancing between certain local structures and the forces

they are exposed to; and is no less inevitable than the other.

§ 314. In all which universal laws, we find ourselves again brought

down to the persistence of force, as the deepest knowable cause of

those modifications which constitute physiological development; as

it is the deepest knowable cause of all other evolution. Here, as

elsewhere, the perpetual lapse from less to greater heterogeneity,

the perpetual begetting of secondary modifications by each primary

modification, and the perpetual approach to a temporary balance on the

way towards a final balance, are necessary implications of the ultimate

fact that force cannot disappear but can only change its form.

It is an unquestionable deduction from the persistence of force, that

in every individual organism each new incident force must work its

equivalent of change; and that where it is a constant or recurrent

force, the limit of the change it works must be an adaptation of

structure such as opposes to the new outer force an equal inner force.

The only thing open to question is, whether such re-adjustment is

inheritable; and further consideration will, I think, show, that to say

it is not inheritable is indirectly to say that force does not persist.

If all parts of an organism have their functions co-ordinated into a

moving equilibrium, such that every part perpetually influences all

other parts, and cannot be changed without initiating changes in all

other parts--if the limit of change is the establishment of a complete

harmony among the movements, molecular and other, of all parts; then

among other parts that are modified, molecularly or otherwise, must

be those which cast off the germs of new organisms. The molecules of

their produced germs must tend ever to conform the motions of their

components, and therefore the arrangements of their components, to the

molecular forces of the organism as a whole; and if this aggregate of

molecular forces be modified in its distribution by a local change

of structure, the molecules of the germs must be gradually changed

in the motions and arrangements of their components, until they are

re-adjusted to the aggregate of molecular forces.

CHAPTER X^A.

THE INTEGRATION OF THE ORGANIC WORLD.

§ 314_a_. That from the beginning of life there has been an

ever-increasing heterogeneity in the Earth’s Flora and Fauna, is

a truth recognized by all biologists who accept the doctrine of

evolution. In discussing the origin of species Mr. Darwin and others

have been mainly occupied in explaining the genesis of now this and

now that form of organism, considered as a member of one or other

series, and regarded as becoming differentiated from its allies. But by

implication, if not avowedly, there has been simultaneously accepted

the belief that the forms continually produced by divergences and

re-divergences, have constituted an assemblage increasingly multiform

in its included kinds. And this, which we are shown by the process of

organic evolution as followed out in its details, is a corollary from

the doctrine of evolution at large, as was pointed out in § 159 of

_First Principles_.

Meanwhile there has been little if any recognition of an accompanying

change, no less fundamental. In the general transformation which

constitutes Evolution, differentiation and integration advance hand

in hand; so that along with the production of unlike parts there

progresses the union of these unlike parts into a whole. Examples of

various kinds before given will recur to the reader, and an addition

to them has just been set forth in the chapter on “Physiological

Integration.” One more example, world-wide in its reach, has still to

be named.

For here it remains to point out that along with the increasing

multiplication of types of organisms covering the Earth’s surface,

there has been ever going on an increasing mutual dependence of

them--an increasing integration of the entire aggregate of living

things.

Many facts which are obvious and many which are quite familiar will

be named as evidence. But I must be excused for reminding the reader

of things that he knows and things that he may easily observe, since,

unless the evidence, trite as it may be, is gathered together and

properly marshalled, the generalization enunciated will not be thought

valid.

§ 314_b_. Respecting the physiological characters of the earliest

forms there is an assumption from which no escape seems possible--the

assumption that they united animal and vegetal characters. Even among

existing microscopic types of the lowest classes, there is such

community of plant-traits and animal-traits that doubts respecting

their proper places in one or the other kingdom are continually

raised--doubts, too, whether, if regarded as vegetal, they are to be

grouped as algoid or fungoid.

Here, however, without entering on moot questions, we may draw the

_à priori_ conclusion that these earliest living things were

double-natured, in so far that they must have had the ability to

assimilate from the inorganic world all the materials of which

protoplasm consists--must therefore, along with the power of

appropriating carbon from its gaseous compound, also have had the power

of appropriating nitrogen, either from one of its combined oxides

or directly from the air with which water is more or less charged.

For before organic substances existed there could have been none but

inorganic sources from which nitrogen could be obtained.

This conclusion concerns us only because it implies homogeneity of

nature in these primordial forms of life. There could not at first

have existed among these minutest of _Protozoa_ even such vague

distinctions as are now presented in a shadowy way by their modern

representatives. And the implication is that during the period

throughout which these smallest, lowest, and simplest living things

alone existed, there could have been, in the absence of kinds, no

mutual dependence.

Since, among various of the lowest types now known to us, the same

individual exhibits a life which is now predominantly vegetal and now

predominantly animal, we cannot err in assuming that there eventually

took place differentiations of this original plant-animal type into

types permanently unlike: some in which the traits were more markedly

vegetal and others in which they were more markedly animal. As fast as

this differentiation arose, there came the beginnings of co-operation

between the predominantly vegetal types which by the aid of light

formed organic matter from the inorganic world, and the predominantly

animal types which, in chief measure, utilized the matter so formed.

Evidently with the rise of such a differentiation came an incipient

mutual dependence. If to the implied algoid type and the animal type

there be added the fungoid type, somewhat intermediate in character,

which in a large proportion of cases lives on the decaying remnants of

the other two, we are furnished with a rude conception of the primary

differentiations and the accompanying vague mutual dependences.

Speculation aside, it suffices to say that early in the history of

life there must have arisen the distinction between _Protozoa_ and

_Protophyta_, and that this distinction foreshadowed that widest

contrast which the higher organic world presents--the contrast

between plants and animals. It is needless to do more than name the

mutual dependence between these two great divisions. That, as being

respectively decomposers of carbon dioxide and exhalers of carbon

dioxide, they act reciprocally, as also in some measure by interchange

of nitrogenous matters; and that the implied general co-operation

serves in an indirect way to unite their lives, and in that sense to

integrate the two kingdoms; needs not to be insisted upon. Further

complications of the mutual dependence will be mentioned by and by. For

the present it suffices to recognize this division of organic functions

as the first which arose and as continuing to be that fundamental one

which more than all others binds organisms at large together.

§ 314_c_. It will be thought by many readers that in speaking of the

contrasted vital activities of plants and animals as constituting a

“division of organic functions,” I am straining words beyond their

meanings; since the conception of organic functions postulates an

organized whole in which they exist, and plants and animals constitute

no such organized whole. But there is at hand an unexpected defence for

this conception--a defence not forthcoming a generation ago, but which

now all biologists will recognize as relevant. I refer to the phenomena

of symbiosis. These present various cases in which the plant-function

and the animal-function are carried on in the same body,--cases in

which the co-operation is not between separate vegetal organisms which

accumulate nutritive matters and separate animal organisms which

consume them, but is a co-operation between vegetal elements and animal

elements forming parts of the same organism.

As introductory to examples of these must first, however, be named an

example of such co-operation between the two great classes of vegetal

organisms--the fungoid and the algoid. Incredible as the statement once

seemed, it is a statement now accepted, that what we know as lichens,

and used to consider as plants forming a certain low class, are now

found to be not plants in the ordinary sense at all, but compound

growths formed of minute algæ and minute fungi, carrying on their lives

together: the algæ furnishing to the fungi certain constituents they

need but cannot directly obtain, and the fungi profiting by certain

materials they obtain from the algæ, either while living or while

individually decaying. Whence it would seem that after the microscopic

vegetal type had become in a large degree differentiated into two main

types, in adaptation to different conditions of life, and had acquired

appropriate specialities of nature, there grew up this communistic

arrangement between certain of them, enabling each to benefit by the

powers which the other had acquired: evidently an exchange of services,

a physiological division of labour, a mutual dependence of functions

analogous to that which exists between functions in an ordinary plant

or animal.

Not differing in principle but only in application, is that symbiosis

above referred to as existing between _Protophyta_ and many _Protozoa_,

as well as between such _Protophyta_ and the lowest kinds of _Metazoa_.

A recent statement that certain amœbæ, made green by contained

chlorophyll, continue to grow and multiply after they have consumed

what nutritive matter may be at hand, is in harmony with various facts

alleged of other _Protozoa_--various other kinds of _Rhizopods_,

various _Heliozoa_, numerous ciliated and flagellated _Infusoria_.

Among _Metazoa_ the like association occurs in one of the sponges,

in the _Hydra viridis_, in various turbellarians, in a rotifer, and

even in two molluscs. In these cases the partnership between the

vegetal cells and the animal cells (existing either as units or as an

organized group such as a polype), is a partnership which, as before,

profits each of the partners--an inference supported by the fact that

_Metazoa_ containing these algoid cells usually place themselves where

the light falls upon them, and can therefore further the production of

the carbo-hydrates which eventually become useful to the animal cells,

while these in some way reciprocate the benefit.

Here, then, we have exchange of services between associated

plant-elements and animal-elements--a performance by them of different

organic functions for the benefit of the aggregate which they unite

to form. Hence, when these vegetal elements and animal elements

are separately embodied in plants and animals, which profit by

one another, we may still properly regard their respective lives

as mutually-dependent organic functions, as said in the preceding

section. We are enabled the better to see how the Earth’s Flora and

Fauna, which are respectively accumulators of motion and expenders of

motion, form mutually-dependent parts of a whole, and are in that sense

integrated. And we shall be prepared to see how all other relations

between organisms which make them subservient one to another, similarly

constitute elements in a general integration of the organic world.

§ 314_d_. Another form of mutual dependence and consequently of

integration is conspicuous--that which accompanied the progressive

increase of size in organisms of the higher classes. We have but to

contemplate the possibilities to see that life must necessarily have

commenced with minute forms, and that the progress to larger ones must

have been by small steps.

For had creatures of appreciable sizes been the first to exist they

would inevitably have disappeared from lack of food. Having no resource

but to devour one another, they would quickly have brought life to an

end. There must have been smaller types serving as prey for larger

ones before these could continue to exist and to multiply: microbes

affording food to infusoria, infusoria affording food to such sized

creatures as the _Entomostraca_, these again supplying food to

small fishes, such as loch-trout, and these last yielding to larger

fishes masses sufficiently great for their needs: each higher grade

requiring lower grades of appropriate bulk. It needs but to ask what

would become of tigers if there were no mammals larger than mice,

to see that the animal world is a linked assemblage, of which the

connected members stand within certain ratios of mass; and that during

the evolution of higher and larger types the linking of grades has

become closer.

That among plants considered as an aggregate relations of like kind,

though far less distinct ones, have all along been growing may be

reasonably concluded. In a world peopled only by microscopic types

there could not have existed the conditions needful for large trees.

Gradual disintegration of rock-surfaces, partly effected by physical

agencies and partly by low forms of plants, had to prepare the way for

superior plants. The production of sufficient soil by mineralogical

decay as well as by the decay of organisms, plant and animal, may be

regarded as having been a preliminary to larger plant-growth; and

though at present the dependence is far less close than that among

animals, yet the benefits yielded to metaphytes by the decomposing

actions carried on by protophytes, as well as those carried on by

microbes permeating the soil, imply a continued general interdependence

throughout the aggregate of plant-forms, apart from more special

interdependences. And then along with this indebtedness of the greater

plants to the smaller during the process of evolution, there must be

named that indebtedness of plant-life to animal-life which Mr. Darwin

has shown in his book on the agency of worms as producers of mould.

§ 314_e_. Services of one to another, and consequent unions, of

more special kinds are infinitely varied, alike within each kingdom

and between the two kingdoms. I refer to those seen in parasitism,

commensalism, and other forms of association. While they do not

conduce to unions of the kind thus far considered, these nevertheless

constitute innumerable links whereby the lives of organisms, plant and

animal, are tied together; sometimes for the advantage of both but in

most cases for the benefit of one to the injury of the other.

Among plants the degrees of dependence are various. Unable to raise

themselves into the air and light, some climb, like the ivy, by

modified rootlets, or spirally coil themselves, or hang by tendrils.

Others there are which gradually strangle the trees they embrace, or

which, like lichens in damp climates, festooning the smaller trees,

by and by cause their decay. Of higher types of epiphytes which

use trees only to gain elevation, the orchids may be instanced.

And then we have plants which, like the mistletoe, fix themselves

on the bark of their hosts, utilizing them partly for purposes of

elevation and partly by appropriation of their juices. After these

may be named those extreme cases in which the parasitic plants,

ceasing to have any chlorophyll-bearing leaves, live wholly on the

juices of the invaded plants. At home the common dodder, and in the

tropics the _Rafflesiaceæ_, belong to this group. There must be

added the numerous forms of minute fungi which in like manner thrive

at the expense of the plants they infest. In all these cases the

interdependence is one-sided, though, as we shall presently see, while

detrimental to one of the two concerned, it is not always detrimental

to the organic world as a whole.

That utilization of one by another among animals which causes immediate

death, is familiar enough in the relations between carnivores and

herbivores. Almost as familiar are those seen in parasitism. Less

familiar are those seen in commensalism; and the least familiar are

those which show us exchange of services. Among these last--the

mutually-beneficial relations--that between the crocodile and the

bird which picks parasites out of its teeth is a striking one; and no

less so is that of the pique-gouffe, an African bird which pierces

the tumour on a buffalo’s back that incloses a parasite. Then of

another kind we have the connexion between aphides and ants: the

one profiting by being carried to better pastures and the other by

increased saccharine excretion. Next comes the class of messmates, the

connexions between some of which are relatively innocent, as witness

the Sea-anemone which settles itself on the shell occupied by a

Hermit-crab, or as witness the _Remora_ fixed on a shark’s skin. Less

innocent is the relation under which one of the two seizes a share

of the food obtained by the other, like the annelid which insinuates

itself between the Hermit-crab and the whelk-shell it inhabits, or like

the small fishes inhabiting certain _Medusæ_, or those which nestle

in the branchial sac of the _Lophius_. After these may be named the

less injurious forms of parasites proper--those which, distinguished

as _Epizoa_, fix themselves on the skins of their hosts, permanently

or temporarily, such as, of the one kind, the _Lernæa_ on fishes, and

of the other kind the Tick on mammals and birds. Then there come the

other class of parasites, most of them highly injurious, distinguished

as _Entozoa_, living within the bodies of their hosts, now in parts of

their alimentary canals, now on other of their mucous surfaces, and now

in various of their organs: these last two groups being so numerous in

their kinds that there are commonly more species than one proper to

each larger animal. One stage further in the complication meets us in

the parasites upon parasites.

But now the general fact, to which these brief indications are

introductory, is that the use made of one organism by another has been

ever widening and becoming more involved. Among plants utilization of

the larger by the smaller--of trees by epiphytes and parasites--must

have arisen since the times when the larger came into existence--times

relatively late in the course of organic evolution. Moreover most

of the plants which utilize others, either by climbing up them or

settling themselves high up on their stems or sucking their juices,

are phænogams, and the plants they utilize are also phænogams; so that

these innumerable interdependences must have been established since

the phænogamic type has become so predominant in respect of both size

and kind. Similarly among animals. Though there are many parasites

belonging, like the Trematodes, to very low classes, there are many

which belong to the _Arthropoda_, and, being degraded forms of that

class, must have come into existence after Arthropods of considerable

structure had been evolved. Again, a large part of the animals infested

by _Epizoa_ and _Entozoa_ are vertebrates--many of the highest types;

and as these are relatively modern all this parasitism must be of

late date. So, too, of much commensalism and many mutually-beneficial

associations. The reciprocal services of ants and aphides must have

originated since the _Hymenoptera_ and _Hemiptera_ became established

types, and since the days when certain insects of the ant-type had

become social, and since the days when aphides had become degraded

members of their order: both dates being relatively recent. And still

more recent must have been the commensalism between the ants and the

many species of other insects which inhabit their nests.

Leaving out relations of the kinds just named, it seems that down from

those between carnivores and their prey to those between lice and their

hosts, such relations profit one of the two species concerned and

injure the other, and that there the matter ends. But it does not end

there; for that multiplication of effects to which people are usually

blind, brings about changes which, as hinted above, though injurious

to the individual are beneficial to the species, and which, when not

beneficial to the species, are often beneficial to the aggregate of

species.

Even where animals of one class live by devouring animals of another

class, we see, on looking beyond the immediate results, certain remote

results that are advantageous. In the first place the process is

one by which inferior individuals--the least agile, swift, strong,

or sagacious--are picked out and prevented from leaving posterity

and lowering the average quality of their kind. At the same time

individuals made feeble by injury or old age, are among those to be

killed and saved from suffering prolonged pains: the evils of death by

disease and starvation being thus limited to the predatory animals,

relatively small in their numbers. Meanwhile a check is put on undue

multiplication. Where a tract of country has been overrun by rabbits,

weasels, thriving on the abundant supply of food, presently become

numerous enough to bring the population of rabbits within moderate

limits; and by doing this benefit not only all those kinds of plants

which are being eaten down, and all those other animals which live on

such plants, but also the rabbits themselves; since, increasing beyond

the means of subsistence, a large part of them would, if not killed,

die of hunger. Between aphides and lady-birds we see a connexion

of like nature: great increase of the first yielding abundant food

to larvæ of the second, ending after a season or so in swarms of

lady-birds, and consequently of their larvæ, whereby the aphides,

immensely diminished, cease so greatly to injure various plants and

the animals dependent on them. Even minute parasites, by the evils

they inflict on one species, profit others: instance the enormous

destruction of flies which a microscopic fungus caused a few years

ago--a destruction which relieved not only man but all the animals

which flies irritate: often so much as to hinder them from feeding.

Such instances remind us how numerous are the bonds by which the lives

of organisms are tied together.

§ 314_f_. I have reserved to the last the clearest and most striking

illustration of this progressing integration throughout the organic

world. I refer to the mutually-beneficial relations established between

plants and animals through the agency of flowers and insects.

Everyone nowadays has been made familiar with the process of

plant-fertilization, and knows that (leaving out of consideration

plants fertilized by wind-borne pollen) the ability to bear seed

depends largely on the aid given by bees, butterflies, and moths. The

exchange of services has been growing ever more various and complicated

during long past periods. We have the acquirement by flowers of bright

colours serving to guide these insects to places where honey is to be

found; and we have their perfumes, also serving for guidance. Then we

have the many different arrangements, often complicated, by which the

visiting insects are obliged to carry away pollen and dust with it the

stigmas of flowers on which they subsequently settle: thus effecting

crossfertilization. _Pari passu_ have gone on insect-developments

made possible by these arrangements and furthering them. Especially

must be named the modification of certain _Hymenoptera_ into

honey-storing bees: the implication being that the entire economy

established by these social insects has been sequent on the growth

of this system of reciprocal benefits. And then, just instancing the

dependence between a particular flower having a long tubular corolla,

and a particular moth having an appropriately long proboscis, it

suffices to say that innumerable specialities of this general relation

everywhere multiply the links by which the vegetal world and the animal

world are here connected. That the effects of the connections tell

largely on the prosperity of both, is suggested by some instances Mr.

Darwin gives, and by a statement recently made in the United States,

by Dr. L. O. Howard, that the greater fostering of bees would much

increase certain of the crops.

But now observe the broad fact to which these few details concerning

plant-fertilization are introductory. All these general and special

relations between plants and animals have arisen since the phænogamic

type came into existence--have, indeed, arisen since the higher members

of that type, the Angiosperms, have appeared; for the Gymnosperms do

not play any part in this intercommunion. But so far as we can judge

of present results of geologic explorations, there were no Angiosperms

during the Eozoic and Paleozoic periods. So that this class of

connexions between animals and vegetals must have been established

since carboniferous times--a period long, indeed, but far shorter than

that which organic evolution at large has occupied.

§ 314_g_. I have but just touched on some salient parts of a subject,

immense in extent and extremely involved, which it would take a volume

to set forth adequately. Enough has been said, however, to indicate the

truth which it is the purpose of the chapter to bring into view and

emphasize--the truth that both of the two great laws of evolution are

exemplified in the organic world as a whole, as they are exemplified in

every organism, and in all other things.

The reader has long since become familiar with the generalization that

while Evolution is a change from the homogeneous to the heterogeneous,

it is also a change from the incoherent to the coherent; and this

change from the incoherent to the coherent has been above exhibited

as going on even throughout that vast assemblage of organisms, plant

and animal, which cover the Earth’s surface. In what we are obliged

to conceive as the earliest stage, when the most minute types of life

alone existed, the aggregate of living things was at once homogeneous

and incoherent. In the course of epochs immeasurable in duration, this

uniform aggregate of beings has been becoming more multiform. And now

we see that instead of forms of life everywhere without the slightest

union caused by mutual dependence, there have slowly arisen forms of

life among which mutual dependences have entailed vital connexions

correspondingly marked. Along with progressing differentiation there

has ever been progressing integration. So that we may recognize

something like a growing life of the entire aggregate of organisms in

addition to the lives of individual organisms--an exchange of services

among parts enhancing the life of the whole.

In this final generalization the law of Evolution is manifested under

its most transcendental form.

PART VI.

LAWS OF MULTIPLICATION.

CHAPTER I.

THE FACTORS.[52]

§ 315. If organisms have been evolved, their respective powers of

multiplication must have been determined by natural causes. Grant that

the countless specialities of structure and function in plants and

animals, have arisen from the actions and reactions between them and

their environments, continued from generation to generation; and it

follows that from these actions and reactions have also arisen those

countless degrees of fertility which we see among them. As in all other

respects an adaptation of each species to its conditions of existence

is directly or indirectly brought about; so must there be directly or

indirectly brought about an adaptation of its reproductive activity to

its conditions of existence.

We may expect to find, too, that permanent and temporary differences

of fertility have the same general interpretation. If the small

variations of structure and function that arise within the limits

of each species, are due to actions like those which, by their

long-accumulating effects, have produced the immense contrasts between

the various types; we may conclude that, similarly, the actions to

which changes in the rate of multiplication of each species are due,

also produce, in great periods of time, the enormous differences

between the rates of multiplication of different species.

Before inquiring in what ways the rapidities of increase are adjusted

to the requirements, both temporary and permanent, it will be needful

to look at the factors. Let us set down first those which belong to the

environment, and then those which belong to the organism.

§ 316. Every living aggregate being one of which the inner actions are

adjusted to balance outer actions, it follows that the maintenance of

its moving equilibrium depends on its exposure to the right amounts of

these actions. Its moving equilibrium may be overturned if one of these

actions is either too great or too small in amount; and it may be so

overturned either by excess or defect of some inorganic agency in its

environment, or by excess or defect of some organic agency.

Thus a plant, constitutionally fitted to a certain warmth and

humidity, is killed by extremes of temperature, as well as by extremes

of drought and moisture. It may dwindle away from want of soil, or

die from the presence of too great or too small a quantity of some

mineral substance which the soil supplies to it. In like manner, every

animal can maintain the balance of its functions so long only as the

environment adds to or deducts from its heat at rates not exceeding

definite limits. Water, too, must be accessible in amount sufficient to

compensate loss. If the parched air is rapidly abstracting its liquid

which there is no pool or river to restore, its functions cease; and

if it is an aquatic creature, drought may kill it either by drying up

its medium or by giving it a medium inadequately aërated. Thus each

organism, adjusted to a certain average in the actions of its inorganic

environment, or rather, we should say, adjusted to certain moderate

deviations from this average, is destroyed by extreme deviations. So,

too, is it with the environing organic agencies. Among plants, only

the parasitic kinds and those united by symbiosis (as well as a few

innocent “lodgers”) depend for their individual preservation on the

presence of certain other organisms (though the presence of certain

other organisms is needful to most plants for the preservation of the

race by aiding fertilization). Here, for the continuance of individual

life, particular organisms must be absent or not very numerous--beasts

that browse, caterpillars that devour leaves, aphides that suck juices.

Among animals, however, the maintenance of the functional balance is

both positively and negatively dependent on the amounts of surrounding

organic agents. There must be an accessible sufficiency of the plants

or animals serving for food; and of organisms that are predatory or

parasitic or otherwise detrimental, the number must not pass a certain

limit.

This dependence of the moving equilibrium in every individual organism

on an adjustment of its forces to the forces of the environment, and

the overthrow of this equilibrium by failure of the adjustment, is

comprehensive of all cases. At first sight it does not seem to include

what we call natural death; but only death by violence, or starvation,

or cold, or drought. But in reality natural death, no less than every

other kind of death, is caused by the failure to meet some outer action

by a proportionate inner action. The apparent difference is due to

the fact that in old age, when the quantity of force evolved in the

organism gradually diminishes, the momentum of the functions becomes

step by step less, and the variations of the external forces relatively

greater; until there finally comes an occasion when some quite moderate

deviation from that average to which the feeble moving equilibrium is

adjusted, produces in it a fatal perturbation.

§ 317. The individuals of every species being thus dependent on certain

environing actions; and severally having their moving equilibria sooner

or later overthrown by one or other of these environing actions; we

have next to consider in what ways the environing actions are so met

as to prevent extinction of the species. There are two essentially

different ways. There may be in each individual a small or great

ability to adjust itself to variations of the agencies around it and to

a small or great number of such varying agencies--there may be little

or much power of preserving the balance of the functions. And there may

be much or little power of producing new individuals to replace those

whose moving equilibria have been overthrown. A few facts must be set

down to enforce these abstract statements.

There are both active and passive adaptations by which organisms

are enabled to survive adverse influences. Plants show us but few

active adaptations: that of the Pitcher-plant and those of the

reproductive parts of some flowers (which do not, however, conduce to

self-preservation) are exceptional instances. But plants have various

passive adaptations; as thorns, stinging hairs, poisonous and acrid

juices, repugnant odours, and the woolliness or toughness that makes

their leaves uneatable. Animals exhibit far more numerous adjustments,

both passive and active. In some cases they survive desiccation, they

hybernate, they acquire thicker clothing, and so are fitted to bear

unfavourable inorganic actions; and they are in many cases fitted

passively to meet the adverse actions of other organisms, by bearing

spines or armour or shells, by simulating neighbouring objects in

colour or form or both, by emitting disagreeable odours, or by having

disgusting tastes. In still more numerous ways they actively contend

with unfavourable conditions. Against the seasons they guard by storing

up food, by secreting themselves in crevices, or by forming burrows

and nests. They save themselves from enemies by developed powers of

locomotion, taking the shape of swiftness or agility or aptitude for

changing their media; by their strength either alone or aided by

weapons; lastly by their intelligence, without which, indeed, their

other superiorities would avail them little. And then these various

active powers serving for defence, become, in other cases, the powers

that enable animals to aggress, and to preserve their lives by the

success of their aggressions.

The second process by which extinction is prevented--the formation of

new individuals to replace the individuals destroyed--is carried on,

as described in the chapter on “Genesis,” by two methods, the sexual

and the asexual. Plants multiply by spontaneous fission, by gemmation,

by proliferation, and by the evolution of young ones from detached

cells and scales and leaves; and they also multiply by the casting

off of spores and sporangia and seeds. In like manner among animals,

there are varied kinds of agamogenesis, from spontaneous fission up to

parthenogenesis, all of them conducing to rapid increase of numbers;

and we have the more familiar process of gamogenesis, also carried

on in a great variety of ways. This formation of new individuals to

replace the old, is, however, inadequately conceived if we contemplate

only the number born or detached on each occasion. There are four

factors, all variable, on which the rate of multiplication depends.

The first is the age at which reproduction commences; the second is

the frequency with which broods are produced; the third is the number

contained in each brood; and the fourth is the length of time during

which the bringing forth of broods continues. There must be taken into

account a further element--the amount of aid given by the parent to

each germ in the shape of stored-up nutriment, continuous feeding,

warmth, protection, &c.: on which amount of aid, varying between

immensely wide limits, depends the number of the new individuals

that survive long enough to replace the old, and perform the same

reproductive process.

Thus, regarding every living organism as having a moving equilibrium

dependent on environing forces, but ever liable to be overthrown by

irregularities in those forces, and always so overthrown sooner or

later; we see that each species of organism can be maintained only

by the generation of new individuals with a certain rapidity, and by

helping them more or less fully to establish their moving equilibria.

§ 318. Such are the factors with which we are here concerned. I have

presented them in abstract shapes for the purpose of showing how they

are expressible in general terms of force--how they stand related to

the ultimate laws of re-distribution of matter and motion.

For the purposes of the argument now to follow, we may, however,

conveniently deal with these factors under a more familiar guise.

Ignoring their other aspects, we may class the factors which affect

each race of organisms as forming two conflicting sets. On the one

hand, by what we call natural death, by enemies, by lack of food, by

atmospheric changes, &c., the race is constantly being destroyed. On

the other hand, partly by the endurance, the strength, the swiftness,

and the sagacity of its members, and partly by their fertility, it

is constantly being maintained. These conflicting sets of factors

may be generalized as--the forces destructive of race and the forces

preservative of race. So generalizing them, let us ask what are the

necessary implications.

CHAPTER II.

_À PRIORI_ PRINCIPLE.

§ 319. The number of a species must at any time be either decreasing

or stationary or increasing. If, generation after generation, its

members die faster than others are born, the species must dwindle

and finally disappear. If its rate of multiplication is equal to its

rate of mortality, there can be no numerical change in it. And if the

deductions by death are fewer than the additions by birth, the species

must become more abundant. These we may safely set down as necessities.

The forces destructive of race must be either greater than the forces

preservative of race, or equal to them, or less than them; and there

cannot but result these effects on number.

We are here concerned only with races that continue to exist; and may

therefore leave out of consideration those in which the destructive

forces, remaining permanently in excess of the preservative forces,

cause extinction. Practically, too, we may exclude the stationary

condition; for the chances are infinity to one against the maintenance

of a permanent equality between the births and the deaths. Hence, our

inquiry resolves itself into this:--In races that continue to exist,

what laws of numerical variation result from these variable conflicting

forces, which are respectively destructive of race and preservative of

race?

§ 320. Clearly if the forces destructive of race, when once in excess,

had nothing to prevent them from remaining in excess, the race would

disappear; and clearly if the forces preservative of race, when once in

excess, had nothing to prevent them from remaining in excess, the race

would go on increasing to infinity. In the absence of any compensating

actions, the only possible avoidance of these opposite extremes would

be an unstable equilibrium between the conflicting forces, resulting

in a perfectly constant number of the species: a state which we know

does not exist, and against the existence of which the probabilities

are, as already said, infinite. It follows, then, that as in every

continuously-existing species, neither of the two conflicting sets

of forces remains permanently in excess; there must be some way of

stopping that excess of the one or the other which is ever occurring.

How is this done? Should any one allege, in conformity with the old

method of interpretation, that there is in each case a providential

interposition to rectify the disturbed balance, he commits himself to

the supposition that of the millions of species inhabiting the Earth,

each one is yearly regulated in its degree of fertility by a miracle;

since in no two years do the forces which foster, or the forces which

check, each species, remain the same; and therefore, in no two years

is there required the same fertility to balance the mortality. Few if

any will say that God continually alters the reproductive activity

of every parasitic fungus and every Tape-worm or _Trichina_, so as

to prevent its extinction or undue multiplication; which they must

say if they adopt the hypothesis of supernatural adjustment. And in

the absence of this hypothesis there remains only one other. The

alternative possibility is, that the balance of the preservative and

destructive forces is self-sustaining--is of the kind distinguished as

a stable equilibrium: an equilibrium such that any excess of one of

the forces at work, itself generates, by the deviation it produces,

certain counter-forces which eventually out-balance it, and initiate an

opposite deviation. Let us consider how, in the case before us, such a

stable equilibrium must be constituted.

§ 321. When a season favourable to it, or a diminution of creatures

detrimental to it, causes any species to become more numerous than

usual, an immediate increase of certain destructive influences takes

place. If it be a plant, the supposed greater abundance itself implies

fuller occupation of the places available for growth--an occupation

which, leaving fewer such places as the multiplication goes on,

becomes a check on further multiplication--itself causes a greater

mortality of seeds that fail to root themselves. And afterwards, in

addition to this passive resistance to continued increase, there comes

an active resistance: the creatures which thrive at the expense of

the species--the larvae, the birds, the herbivores--increase too. If

it be an animal that has grown more numerous, then, unless by some

exceptional coincidence a simultaneous and proportionate addition to

the animals or plants serving for food has occurred, there must result

a relative scarcity of food. Enemies, too, be they beasts of prey or

be they parasites, must quickly begin to multiply. Hence, each kind

of organism, previously existing in something like its normal number,

cannot have its number raised without a rise of the destructive forces,

negative and positive, quickly commencing. Both negative and positive

destructive forces must augment until this increase of the species is

arrested. The competition for places on which to grow, if the species

be vegetal, or for food if it be animal, must become more intense as

the over-peopling of the habitat progresses; until there is reached

the limit at which the mortality equals the reproduction. And as, at

the same time, enemies will multiply with a rapidity which soon brings

them abreast of the augmented supply of prey, the positive restraint

they exert will help to bring about an earlier arrest of the expansion

than pressure of population alone would cause. One more inference may

be drawn. Had the species to meet no repressing influence save that

negative one of relatively-diminished space or relatively-diminished

food-supply, the cause leading to its increase might carry it up

to the limit set by this, and there leave it: its enlarged number

might be permanent. But the positive repressing influence that has

been called into existence, will prevent this. For the increase of

enemies, commencing, as it must, after the increase of the species,

and advancing in geometrical progression until it is itself checked

in like manner, will end in an excess of enemies. Whereupon must

result a mortality of the species greater than its multiplication--a

decrease which will continue until its habitat is under-peopled, its

unduly-numerous enemies decimated by starvation, and the destroying

agencies reduced to a minimum. Whence will follow another increase.

Thus, as before indicated (_First Prin._ §§ 85, 173), there is

here, as wherever antagonistic forces are in action, an alternate

predominance of each, causing a rhythmical movement--a rhythmical

movement which constitutes a moving equilibrium in those cases where

the forces are not dissipated with appreciable rapidity, or are

re-supplied as fast as they are dissipated. While, therefore, on the

one hand, we see that the continued existence of a species necessarily

implies some action by which the destructive and preservative forces

are self-adjusted; we see, on the other hand, that such an action is an

inevitable consequence of the universal process of equilibration.

§322. Is this the sole equilibration which must exist? Clearly not.

The temporary compensating adjustments of multiplication to mortality

in each species, are but introductory to the permanent compensating

adjustments of multiplication to mortality among species in general.

The above reasoning would hold just as it now does, were all species

equally prolific and all equally short-lived. It yields no answer

to the inquiries--why do their fertilities differ so enormously,

or why do their mortalities differ so enormously? and how is the

general fertility adapted to the general mortality in each? The

balancing process we have contemplated can go on only within moderate

limits--must fail entirely in the absence of a due proportion

between the ordinary birth-rate and the ordinary death-rate. If the

reproduction of mice proceeded as slowly as the reproduction of men,

mice would be extinct before a new generation could arise: even did

their natural lives extend to fifteen or sixteen years, it would still

be extremely improbable that any would for so long survive all the

dangers they are exposed to. Conversely, did oxen propagate as fast

as infusoria, the race would die of starvation in a week. Hence, the

minor adjustment of varying multiplication to varying mortality in each

species, implies some major adjustment of average multiplication to

average mortality. What must this adjustment be?

We have already seen that the forces preservative of race are

two--ability in each member of the race to preserve itself, and ability

to produce other members--power to maintain individual life, and power

to generate the species. These must vary inversely. When, from lowness

of organization, the ability to contend with external dangers is

small, there must be great fertility to compensate for the consequent

mortality; otherwise the race must die out. When, on the contrary, high

endowments give much capacity of self-preservation, a correspondingly

low degree of fertility is requisite. Given the dangers to be met as a

constant quantity; then as the ability to meet them must be a constant

quantity too; and as this is made up of the two factors, power to

maintain individual life and power to multiply, these cannot do other

than vary inversely: one must decrease as the other increases.

It needs but to conceive the results of nonconformity to this law,

to see that every species must either conform to it or cease to

exist. Suppose, first, a species whose individuals, having but small

self-preservative powers, are rapidly destroyed, to be at the same

time without reproductive powers proportionately great. The defect

of fertility, if extreme, will result in the death of one generation

before another has grown up. If less extreme, it will entail a

scarcity such that in the next generation sexual congress will be too

infrequent to maintain even the small number which remains; and the

race will dwindle with increasing rapidity. If still less extreme, the

consequent degree of sparseness, while not so great as to prevent an

adequate number of procreative unions, will be so great as to render

special food abundant and special enemies few--will thus diminish the

destructive forces so much that the self-preservative forces will

become _relatively_ great: so great, relatively, that when combined

with the small ability to propagate the species, they will suffice to

balance the small destructive forces. Suppose, next, a species whose

individuals have high powers of self-preservation, while they have

powers of multiplication much beyond what is needful. The excess of

fertility, if extreme, will cause sudden extinction of the species by

starvation. If less extreme, it must produce a permanent increase in

the number of the species; and this, followed by intenser competition

for food and augmented number of enemies, will involve such an increase

of the dangers to individual life, that the great self-preserving

powers of the individuals will not be more than sufficient to cope with

them. That is to say, if the fertility is relatively too great, then

the ability to maintain individual life inevitably becomes smaller,

_relatively_ to the requirements; and the inverse proportion is thus

established.

So that when, from comparing the different states of the same species,

we go on to compare the states of different species, we see that there

is an analogous adjustment--analogous in the sense that great mortality

is associated with great multiplication, and small mortality with small

multiplication. And we see that the unlikeness of the cases consists

merely in this, that what is a temporary relation in the one is a

permanent relation in the other.

§ 323. For the moment it does not concern us to inquire what is the

origin of this permanent relation. That which we have now to note, is

simply that in some way or other there must be established an inverse

proportion between the power to sustain individual life and the power

to produce new individuals. Whether or not this permanent relation is

self-adjusting in long periods of time, as the temporary relation is

self-adjusting in short periods of time, is a separate question. The

purpose of this chapter is fulfilled by showing that such a permanent

relation must exist.

But having recognized the _à priori_ principle that in races which

continuously survive, the forces destructive of race must be

equilibrated by the forces preservative of race; and that, supposing

these are constant, there must be an inverse proportion between

self-preservation and race-preservation; we may go on to inquire how

this relation, necessary in theory, arises in fact. Leaving out the

untenable hypothesis of a supernatural pre-adjustment, we have to ask

in what way an adjustment comes about as a result of Evolution. Is it

due to the survival of varieties in which the proportion of fertility

to mortality happens to be the best? Or is the fertility adapted to the

mortality in a more direct way? To these questions let us now address

ourselves.

CHAPTER III.

OBVERSE _À PRIORI_ PRINCIPLE.

§ 324. When dealing with its phenomena inductively, we saw that however

it may be carried on, Genesis “is a process of negative or positive

disintegration; and is thus essentially opposed to that process of

integration which is the primary process in individual evolution.”

(§ 76.) Each new individual, whether separated as a germ or in some

more-developed form, is a deduction from the mass of a pre-existing

individual or of two pre-existing individuals. Whatever nutritive

matter is stored-up along with the germ, if it be deposited in the

shape of an egg, is so much nutritive matter lost to the parent.

No drop of blood can be absorbed by the fœtus, nor any draught of

milk sucked by the young when born, without taking from the mother

tissue-forming and force-evolving materials to an equivalent amount.

And all subsequent supplies given to progeny, if they are nurtured,

involve, to a parent or parents, so much waste in exertion which does

not bring its return in assimilated food.

Conversely, the continued aggregation of materials into one organism,

renders impossible the formation of other organisms out of those

materials. As much assimilated food as is united into a single whole,

is so much assimilated food withheld from a plurality of wholes which

might else have been produced. Given the absorbed nutriment as a

constant quantity, and the longer the building of it up into a concrete

shape goes on, the longer must be postponed any building of it up into

discrete shapes. And, similarly, the larger the proportion of matter

consumed in the functional actions of parents, the smaller must be the

proportion of matter which can remain to establish and support the

functional actions of offspring.

Though the necessity of these universal relations is tolerably obvious

as thus stated generally, it will be useful to dwell for a brief space

on their leading aspects.

§ 325. That disintegration which constitutes genesis, may be such as

to disperse entirely the aggregate which integration has previously

produced--the parent may dissolve wholly into progeny. This dissolution

of each aggregate into two or many aggregates, may occur at very short

intervals, in which case the bulk attained can be but extremely small;

or it may occur at longer intervals, in which case a larger bulk may be

attained.

Instead of quickly losing its own individuality in the individualities

of its offspring, each member of the race may, after growing for a

time, have portions of its substance begin to develop into the parental

shape and presently detach themselves; and the parent, maintaining its

own identity, may continue indefinitely so to produce young ones. But

clearly, the earlier it commences doing this, and the more rapidly it

does it, the sooner must the increase of its own bulk be stopped.

Or again, growth and development continuing for a long period without

any deduction of materials, an individual of considerable size and

organization may result; and then the abstraction of substance for

the formation of new individuals, or rather the eggs of them, may

be so great that as soon as the eggs are laid the parent dies of

exhaustion--dies, that is, from an excessive loss of the nutritive

matters needed for its own activities.[53]

Once more, the deduction of materials for the propagation of the

species may be postponed long enough to allow of great bulk and complex

structure being attained. The procreative subtraction then setting

in, while it checks and presently stops growth, may be so moderate

as to leave vital capital sufficient to carry on the activities of

the parent; may go on as long as parental vigour suffices to furnish,

without fatal result, the materials needed to produce young ones; and

may cease when such a surplus cannot be supplied, leaving the parental

life to continue.

§ 326. The opposite side of this antagonism has also several aspects.

Progress of organic evolution may be shown in increased bulk, in

increased structure, in increased amount or variety of action, or

in combinations of these; and under any of its forms this carrying

higher of each individuality, implies a correlative retardation in the

establishment of new individualities.

Other things equal, every normal addition to the bulk of an organism

is an augmentation of its life.[54] Besides being an advance in

integration, it implies a greater total of activities gone through in

the assimilation of materials; and it implies, thereafter, a greater

total of the vital changes taking place from moment to moment in all

parts of the enlarged mass. Moreover, while increased size is thus,

in so far, the expression of increased life, it is also, where the

organism is active, the expression of increased ability to maintain

life--increased strength. Aggregation of substance is almost the only

mode in which self-preserving power is shown among the lowest types;

and even among the highest, sustaining the body in its integrity

is that in which self-preservation fundamentally consists--is the

end which the widest intelligence is indirectly made to subserve.

While, on the one hand, the increase of tissue constituting growth is

conservative both in essence and in result; on the other hand, decrease

of tissue, either from injury, disease, or old age, is in both essence

and result the reverse. And if so, every addition to individual life

thus implied, necessarily delays or diminishes the casting off of

matter to form new individuals.

Other things equal, too, a greater degree of organization involves

a smaller degree of that disorganization shown by the separation

of reproductive gemmæ and germs. Detachment of a living portion or

portions from what was previously a living whole, is a ceasing of

co-ordination; and is therefore essentially at variance with that

establishment of greater co-ordination which is achieved by structural

development. In the extreme cases where a living mass is continually

dividing and subdividing, it is manifest that there cannot arise

much physiological division of labour; since progress towards mutual

dependence of parts is prevented by the parts becoming independent.

Contrariwise, it is equally clear that in proportion as the

physiological division of labour is carried far, the separative process

must be localized in some comparatively small portion of the organism,

where it may go on without affecting the general structure--must

become relatively subordinate. The advance that is shown by greater

heterogeneity, must be a hindrance to multiplication in another way.

For organization entails cost. That transfer and transformation

of materials implied by differentiation, can be effected only by

expenditure of force; and this supposes consumption of digested and

absorbed food, which might otherwise have gone to make new organisms,

or the germs of them. Hence, that individual evolution which consists

in progressive differentiation, as well as that which consists in

progressive integration, necessarily diminishes that species of

dissolution, general or local, which propagation of the race exhibits.

In active organisms we have yet a further opposition between

self-maintenance and maintenance of the race. All motion, sensible and

insensible, generated by an animal for the preservation of its life,

is motion liberated from decomposed nutriment--nutriment which, if

not thus decomposed, would have been available for reproduction; or

rather--might have been replaced by nutriment fitted for reproductive

purposes, absorbed from other kinds of food. Hence, in proportion as

the activities increase--in proportion as, by its more varied, complex,

rapid, and vigorous actions, an animal gains power to support itself

and to cope with surrounding dangers, it must lose power to propagate.

§ 327. How may this antagonism be best expressed in a brief way? If

self-preservation displayed itself in the highest organisms, as it

does in the lowest, in little else but continuous growth; and if

race-preservation consisted always, as it does often, of nothing beyond

detachment of portions from the parental mass; then the antagonism

would be, throughout, the obviously-necessary one of integration and

disintegration. Maintenance of the individual and propagation of the

species, being respectively aggregative and separative, it would be

as self-evident that they vary inversely, as it is self-evident that

addition and subtraction undo one another. But though the simplest

types show us the opposition of self-maintenance and race-maintenance

almost wholly under this form; and though higher types, up to the

most complex, exhibit it to a great extent under this form; yet, as

we have just seen, this is not its only form. The total material

monopolized by the individual and withheld from the race, must be

stated as the quantity united to form its fabric, _plus_ the quantity

expended in differentiating its fabric, _plus_ the quantity expended

in its self-conserving actions. Similarly, the total material devoted

to the race at the expense of the individual, includes that which is

directly subtracted from the parent in the shape of egg or fœtus,

_plus_ that which is directly subtracted in the shape of milk, _plus_

that which is indirectly subtracted in the shape of matter consumed

in exertions for fostering the young. Hence this inverse variation

is not expressible in simple terms of aggregation and separation. As

we advance to more highly-evolved organisms, the total cost of an

individual becomes very much greater than is implied by the amount of

tissue composing it. So, too, the total cost of producing each new

individual becomes very much greater than that of its mere substance.

And it is between these two total costs that the antagonism exists.