This parallel, only just traceable among these Rhizopods, which are

perpetually changing the distribution of their outer substance, becomes

at once marked in those higher _Protozoa_ which have fixed shapes,

and maintain constant relations between their surfaces and their

environments. Indeed the Rhizopods themselves, on passing into a state

of quiescence in which the relations of outer and inner parts are

fixed, become encysted: there is formed a hardened outer coat different

from the matter which it contains. And what is here a temporary

character answering to a temporary definiteness of conditions, is

in the _Infusoria_ a constant character, answering to definite

conditions that are constant. Each of these minute creatures, though

not coated by a distinct membrane, has an outer layer of excreted

substance forming a delicate cuticle.

§ 288. The early establishment of this primary contrast of tissues

answering to this primary contrast of conditions, is no less

conspicuous in aggregates of the second order. The feebly-integrated

units of a Sponge, with individualities so little merged in that of

the whole they form that most of them still retain their separate

activities, nevertheless show us, in the unlikeness that arises between

the outermost layer and the contained mass, the effect of converse with

unlike conditions. This outermost layer is composed of units somewhat

flattened and united into a continuous membrane--a kind of rudimentary

skin.

Secondary aggregates in which the lives of the units are more

subordinate to the life of the whole, carry this distinction further.

The leading physiological trait of every cœlenterate animal is the

divisibility of its substance into endoderm and ectoderm--the part

next the food and the part next the environment. Fig. 147 (§ 201),

representing a portion of the body-wall of a _Hydra_ seen in section,

gives some idea of this fundamental differentiation. The creature

consists of a simple sac, the cavity of which is in communication with

the surrounding water; and hence the unlikeness between the outer and

inner layers has not become great. The essential contrast is that

between the differentiated parts of what was originally the same

part--a uniform membrane composed of juxtaposed cells.

For here, indeed, we are shown unmistakably how the primary contrast

of structures follows upon the primary contrast of conditions. The

ordinary form from which low types of the _Metazoa_ set out, is a

hollow sphere formed of cells packed side by side--a blastula, as it

is called: all these cells being similarly exposed to the environment.

The blastula presently changes into what is called a gastrula--a form

resulting from the introversion of one of the sides of the blastula.

If there be taken a small ball of vulcanized india-rubber, say an

inch or more in diameter, and having a hole in it through which the

air may escape, and if one side of it be thrust inwards so as to

produce a cup, and if the wide opening of the cup be supposed to

contract, thus becoming a narrow opening, there will result something

like the gastrula form. Manifestly that part of the original layer

which has become internal is differently conditioned from the rest

which remains external: the one continuing to hold converse with the

forces of the environment, while the other begins to hold converse

with the nutritive matters taken into the sac-formed chamber--the

archenteron or primitive stomach. Interesting evidence of the primitive

externality of the digestive cavity is yielded by the fact that whereas

the blastula consisted of ciliated cells, and whereas the ciliation

persists throughout life on the outer layer, or parts of it, in sundry

low types--even in some Chætopods--it persists also on the alimentary

tract of sundry low types: not only in the _Hydra_ but commonly in

Nemertines, in some _Platyhelminthes_, and even in some leeches.

Besides being enabled thus to understand how an aggregate of

_Amœba_-form units, originally consisting of a single layer, may pass

into an aggregate consisting of a double layer; we may also understand

under what influences the transition takes place. If the habit which

some of the primary aggregates have, of wrapping themselves round

masses of nutriment, is followed by a secondary aggregate, there will

naturally arise just that re-differentiation which the _Hydra_ shows us.

§ 289. This account of the primary differentiation carries us only

half-way towards a true conception of the distinction between outer

and inner tissues. Though, using words in their current senses, this

introverted part of the primitive layer has become internal in contrast

with the remainder, which continues external, yet this introverted

part has not become internal in the strict physiological sense. For

it remains subject to the actions of those environing matters which

are taken in as food: such environing matters, when they happen to be

moving prey, acting upon it much as they might act upon the exterior.

So that this introverted part has a quasi-externality. It has not

the same absolute internality as have those parts which never come

in contact with products of the outer world. Here we must briefly

recognize the distinction between these parts and the parts thus far

considered.

Reverting to our symbol, the india-rubber ball, it will be seen that

the introversion may be so complete that the cavity is obliterated,

with the result that the internal surfaces of the outer and inner

layers come in contact. This is the state reached in the simplest

cœlenterate animal, the _Hydra_: there being in it nothing more

than a thin structureless lamella between the ectoderm and endoderm,

as shown in Fig. 147. This lamella represents all that there is of

strictly internal tissues. But the introversion, instead of bringing

the inner surfaces of the ball into contact, may be so far incomplete

as to leave a space, and in various creatures and embryos of others,

symbolized by this arrangement, this space becomes occupied by a tissue

formed from one or other or both of the two primary tissues--the

mesoblast or mesoderm. This intermediate layer, sometimes, as in the

_Medusa_, growing into a mass of jelly serving as a fulcrum for

the creature’s contractions, or, as in the Sponge, giving a passive

basis to the active tissues, becomes in higher animals the layer out

of which the structures that support the body and move it about, as

well as those that distribute prepared nutriment, are developed. From

it arise the bones, the muscles, and the vascular system--the masses

of differentiated tissue which are truly internal and occupy what is

called the body-cavity or peri-visceral space.

In the higher types of animals this space comes to be partially

occupied by a structure that may be described as a cavity within

a cavity--the cœlom. Most zoologists regard this as arising by a

re-introversion of the _archenteron_ or primary alimentary sac. It

is easily to be perceived that after the introversion which produces

this digestive cavity, the wall of the cavity may be again introverted

in such way as to intrude into the peri-visceral space. The cœlom thus

formed is subsequently shut off. Becoming included among the more

truly internal structures, and in part giving origin to certain lining

membranes, it has for its chief function the formation of organs for

the excretion and emission of nitrogenous waste and of the generative

products: some portions of it retaining, as a consequence, indirect

connexions with the environment and characters usually accompanying

such connexions.

Here we are not concerned with further details: the aim being simply to

indicate the way in which out of the original layer, wholly external,

there arise, by primary and secondary introversions, and the formation

of intermediate membranes and spaces, the chief contrasts between

outer and inner tissues, and how there simultaneously go on the

differentiations accompanying different conditions.

§ 289_a_. Another all-important differentiation between outer tissues

and inner tissues has now to be set forth--that by which the nervous

system becomes established and distinguished. Strangely enough, like

the one above described, it is sequent upon an introversion: the

nervous system is primarily a skin-structure and develops by the

infolding of this skin-structure.

In creatures possessing the earliest rudiments of nerves these exist

in certain superficial cells. Each has a small tubular orifice from

which projects a minute hair, and each has on its under side processes

running into the tissue below, and serving, as it seems, to conduct

impressions from the projecting hair when it is disturbed by contacts

with foreign bodies. A plexus of fibres bringing the inner processes

of such cells into communication arises, and forms something like

a nervous layer capable of propagating impulses in all directions.

At a subsequent stage some of the superficial cells, ceasing to be

themselves the recipients of external stimuli, sink inwards and become

ganglion-cells connected with the nervous plexus--agents, as we must

suppose, for the reception, multiplication, and diffusion of the

impulses received from the outer cells.

As thus far developed, the nervous structure is one fitted only

for a vague stimulation of dispersed contractile fibres, causing

movements of an undirected kind. A concentration of these superficial

nervous structures is a probable preliminary to the next change--an

all-important change. For a part of the surface begins to sink inwards,

forming, in the _Vertebrata_, a groove; and from the lining cells

of this groove, which presently closes over, the central parts of

the nervous system arise: definite nerves having meantime, as we may

suppose, been developed out of the indefinite nervous plexus.

Neglecting what there is in this of a speculative nature, it is

sufficient for the present purpose to recognize the undoubted fact that

the nervous system is developed from the ectoderm, and that, originally

external, it is made internal by a process of sinking in or by a

process of definite introversion.

§ 290. Whether direct equilibration or indirect equilibration has had

the greater share in producing these fundamental contrasts between

the inner and outer tissues of animals, must be left undecided. The

two causes have all along co-operated--modification of the individual

accumulated by inheritance predominating in some cases, and in other

cases modification of the race by survival of the incidentally fittest.

On the other hand, the action of the medium on the organism cannot fail

to change its surface more than its centre, and so differentiate the

two; while, on the other hand, the surfaces of organisms inhabiting

the same medium display extreme unlikenesses which cannot be due to

the immediate actions of their medium. Let us dwell a moment on the

antithesis.

We have abundant evidence that animal protoplasm is rapidly modified by

light, heat, air, water, and the salts contained in water--coagulated,

turned from soluble into insoluble, partially changed into isomeric

compounds, or otherwise chemically altered. Immediate metamorphoses

of this kind are often obviously produced in ova by changes of their

media. At the outset, therefore, before yet there existed any such

differentiation as that which now usually arises by inheritance,

these environing agencies must have tended to originate a protective

envelope. For a modification produced by them on the superficial part

of the protoplasm, must either have been a decomposition or else the

formation of a compound which remained stable under their subsequent

action. There would be generated an outer layer of substance that was

so molecularly immobile as to be incapable of further metamorphoses,

while it would shield the contained protoplasm from that too-great

action of external forces which, by rapidly changing the unstable

equilibrium of its molecules into a relatively stable equilibrium,

would arrest development. Evidently organic evolution, whether

individual or general, must always and everywhere have been subordinate

to these physical necessities. Though natural selection, beginning with

minute portions of protoplasm, must all along have tended to establish

a molecular composition apt to undergo this differentiation of surface

from centre to the most favourable extent, yet it must all along have

done so while controlled by this process of direct equilibration.

Contrariwise, the many and great unlikenesses among the dermal

structures of creatures inhabiting the same element, cannot be ascribed

to any such cause. The contrasts between naked and shelled Gastropods,

between marine Worms and Crustaceans, between soft-skinned Fishes

and Fishes in armour like the _Pterichthys_, must have been produced

entirely by natural selection. Environing forces are, as before, the

ultimate causes; but the forces are now not so much those exercised by

the medium as those exercised by the other inhabitants of the medium;

and they do not act by modifying the surface of the individual, but

by killing off individuals whose surfaces are least fitted to the

requirements: thus slowly affecting the species. Still the dermal

skeleton bristling with spines, which protects the _Diodon_ or the

_Cyclichthys_ from enemies it could not escape, comes within the

general formula of an outer tissue differentiated from inner tissues by

the outer actions to which the creature is exposed: the differentiation

having gone on until there is equilibrium between the destructive

forces to be met and the protective forces which meet them.

If we venture to apportion the respective shares which mediate and

immediate actions have had in differentiating outer from inner tissues,

we shall probably not be far wrong in ascribing that part of the result

which is alike in all animals, mainly to the direct actions of their

media, while we ascribe the multitudinous unlikenesses of the results

in various animals, partly to the indirect actions of the media, and

partly to the indirect actions of other animals by which the media are

inhabited. That is to say, while assigning the specialities of the

differentiations to the specialities of converse with the agencies in

the environment, most of them organic, we may assign to the constant

and universal converse with its inorganic agencies, the universal

characteristic of tegumentary structures--their growth outwards from a

layer lying below the surface which continually produces new substance

to replace the substance worn away or cast off.

Here let me add a piece of evidence which strengthens the general

argument, at the same time that it justifies this apportionment. When

ulceration has gone deep enough to destroy the tegumentary structures,

these are never reproduced. The puckered surface formed where an ulcer

heals, or where a serious burn has destroyed the skin, consists of

modified connective tissue, which, as the healing goes on, spreads

inwards from the edges of the ulcer: some of it, perhaps, growing from

the portions of connective tissue that dip down between the muscular

bundles. This connective tissue is normally covered by the epidermis

and thus sheltered from environing actions. What has happened to it?

It has now become the outermost layer. And how does it comport itself

under its new conditions? It produces a superficial substance which

plays the part of the epidermis and grows outwardly. For since the

surface, subject to friction and exfoliation, has to be continually

renewed, there must be a continual reproduction of an outermost layer

from a layer beneath. That is to say, the contact of this deep-seated

tissue with outer agencies, produces in it some approach towards

that character which we find universally characterizes outer tissue.

But while we see under this exposure to the conditions common to

all integument, a tendency to assume the structure common to all

integument, we see no tendency to assume any of the specialities of

tegumentary structure: no rudiments of glands or hair sacs make their

appearance.

Analogous conclusions may be drawn respecting the processes of

differentiation by which from the outer layer nervous tissue and

finally a nervous system are evolved. Here, also, both direct and

indirect equilibration appear to have operated. Two reasons may be

assigned for the belief that the transformation of certain superficial

cells into sensitive cells was initiated by exposure to external

stimuli. The first is that, extremely unstable as protoplasm is,

disturbances received by the outer side of a specially-exposed cell

could scarcely fail to cause changes passing through it towards the

interior mass of the body, and that perpetual repetition of such

changes would tend to generate channels of easy transmission through

the protoplasm. The second reason is that, if we do not assume this

process of initiation but assume that survival of the fittest was

the sole agency, then no reason can be assigned why the nervous

system should not have been at the outset formed internally instead

of being initiated externally and then transferred to the interior:

the roundabout process would be inexplicable. At the same time the

production of a central nervous system by introversion of superficial

sensitive cells cannot be ascribed to the differentiating effects

of external stimuli, but must be ascribed to natural selection. No

perpetual repetition of outer disturbances would cause the sinking

inwards, and covering up, of the specially-sensitive area and

the plexus below it. But it is manifest that since these nervous

structures, at once all-important and easily injured, would be safer

if removed from the surface, survival of the fittest, continually

preserving those in which they were more deeply seated, would tend to

produce an arrangement in which all parts but the actual receivers of

external stimuli became internal.

Hence, contemplating generally these two fundamental differentiations

of inner from outer tissues, we may conclude that though their first

stages resulted from direct equilibration, their subsequent and higher

stages resulted from indirect equilibration.

CHAPTER VII.

DIFFERENTIATIONS AMONG THE OUTER TISSUES OF ANIMALS.

§ 291. The outer tissues of animals, originally homogeneous over their

whole surfaces, pass into a heterogeneity which fits their respective

parts to their respective conditions. So numerous and varied are the

implied differentiations, that it is impracticable here to deal with

them all even in outline. To trace them up through classes of animals

of increasing degrees of aggregation, would carry us into undue detail.

Did space permit, it would be possible to point out among the

_Protozoa_, various cases analogous to that of the _Arcella_; which may

be described as like a microscopic Limpet, having a sarcode body of

which the upper surface has become horny, while the lower surface with

its protruding pseudopodia, retains the primitive jelly-like character.

That differentiations of this kind have been gradually established

among these minute creatures through the unlike relations of their

parts to the environment, is an inference supported by a form which,

while the rest of the body has a scarcely distinguishable coating,

“agrees with _Arcella_ and _Difflugia_ in having the pseudopodia

protrusible from one extremity only of the body.”

Many parallel specializations of surface among aggregates of the second

order might be instanced from the _Cœlenterata_. In the _Hydra_, the

ectoderm presents over its whole area no conspicuous unlikenesses;

but there usually exist in the hydroid polypes of superior types,

decided contrasts between the higher and lower parts. While the

higher parts retain their original characters, the lower parts excrete

hard outer layers yielding support and protection. Various stages

of the differentiation might be followed. “In _Hydractinia_,” says

Prof. Green, this horny layer “becomes elevated at intervals to form

numerous rough processes or spines, while over the general surface

of the ectoderm its presence is almost imperceptible.” In other

types, as in _Cordylophora_, it spreads part way up the animal’s

sides, ending indefinitely. In _Bimeria_ it “extends itself so as to

enclose the entire body of each polypite, leaving bare only the mouth

and tips of the tentacles.” While in _Campanularia_ it has become a

partially-detached outer cell, into which the creature can retract its

exposed parts.

But it is as needless as it would be wearisome to trace through the

several sub-kingdoms the rise of these multiform contrasts, with the

view of seeking interpretations of them. It will suffice if we take a

few groups of the illustrations furnished by the higher animals.

§ 292. We may begin with those modifications of surface which subserve

respiration. Though we ordinarily think of respiration as the quite

special function of a quite special organ, yet originally it is not

so. Little-developed animals part with their carbonic acid and absorb

oxygen, through the general surface of the body. Even in the lower

types of the higher classes, the general surface of the body aids

largely in aërating the blood; and the parts which discharge the

greater part of this function are substantially nothing more than

slightly altered and extended portions of the skin.

Such differentiations, marked in various degrees, are to be seen among

_Mollusca_. In the _Pteropoda_ the only modification which appears to

facilitate respiration, is the minute vascularity of one part of the

skin. Higher types possess special skin-developments. The _Doris_ has

appendages developed into elaborately-branched forms--small trees of

blood-vessels covered by slightly-changed dermal tissues. And these

arborescent branchiæ are gathered together into a single cluster. Thus

there is evidence that large external respiratory organs have arisen

by degrees from simple skin: as, indeed, they do arise during the

development of each individual having them. Just as gradually as in

the embryo a simple bud on the integument, with its contained vascular

loop, passes by secondary buddings into a tree-like growth penetrated

everywhere by dividing and subdividing blood-vessels; so gradually has

there probably proceeded the differentiation which has turned part

of the outer surface into an organ for excreting carbonic acid and

absorbing oxygen.

Certain inferior vertebrate animals present us with a like

metamorphosis of tissues. These are the _Amphibia_. The branchiæ

here developed from the skin, are covered with cellular epidermis, not

much thinner than that covering the rest of the body. Like it they

have their surfaces speckled with pigment-cells; and are not even

conspicuous by their extra vascularity--where they are temporary at

least. They facilitate the exchange of gases in scarcely any other

way than by affording a larger area of contact with the water, and

interposing a rather thinner layer of tissue between the water and the

blood-vessels. Those very simple branchiæ of the larval _Amphibia_

that have them but for a short time, graduate into the more complex

ones of those that have them for a long time or permanently; showing,

as before, the small stages by which this heterogeneity of surface

accompanying heterogeneity of function may arise.

In what way are such differentiations established? Mainly, no doubt, by

natural selection; but also to some degree, I think, by the inheritance

of direct adaptations. That a portion of the integument at which

aëration is favoured by local conditions, should thereby be led to grow

into a larger surface of aëration, appears improbable. Survival of

those individuals which happen to have this portion of the integument

somewhat more-developed, seems here the only likely cause.

§ 293. Among the conspicuous modifications by which the

originally-uniform outer layer is rendered multiform, are the

protective structures. Let us look first at the few cases in which the

formation of these is ascribable mainly to direct equilibration.

Already reference has been more than once made to those thickenings

that occur where the skin is exposed to unusual pressure and friction.

Are these adaptations inheritable? and may they, by accumulation

through many generations, produce permanent dermal structures fitted

to permanent or frequently-recurring stress? Take, for instance, the

callosities on the knuckles of the _Gorilla_, which are adapted to its

habit of partially supporting itself on its closed hands when moving

along the ground. Shall we suppose that these defensive thickenings are

produced afresh in each individual by the direct actions; or that they

are inherited modifications caused by such direct actions; or that they

are wholly due to the natural selection of spontaneous variations? The

last supposition does not seem a probable one. Such thickenings, if

spontaneous, would be no more likely to occur on the knuckles than on

any other of the hundred equal areas forming the skin-surface at large;

and the chances against their simultaneous occurrence on all eight

knuckles would be incalculable. Moreover, the implication would be that

those slight extra thicknesses of skin on the knuckles, with which we

must suppose the selection to have commenced, were so advantageous

as to cause survivals of the individuals having them, in presence of

other superiorities possessed by other individuals. Then that survivals

so caused, if they ever occurred at all, should have occurred with

the frequency requisite to establish and increase the variation, is

hardly supposable. And if we reject, as also unlikely, the reproduction

of these callosities _de novo_ in each individual (for this would

imply that after a thousand generations each young gorilla began with

knuckles having skin no thicker than elsewhere), there remains only the

inference that they have arisen by the transmission and accumulation of

functional adaptations. Another case which seems interpretable only in

an analogous way, is that of the spurs that are developed on the wings

of certain birds--on those of the Chaja screamer for example. These are

weapons of offence and defence. It is a familiar fact that some birds

strike with their wings, often giving severe blows; and in the birds

named, the blows are made more formidable by the horny, dagger-shaped

growths standing out from those points on the wings which deliver

them. Are these spurs directly or indirectly adaptive? To conclude

that natural selection of spontaneous variations has caused them, is

to conclude that, without any local stimulus, thickenings of the skin

occurred symmetrically on the two wings at the places required; that

such thickenings, so localized, happened to arise in birds given to

using their wings in fight; and that on their first appearance the

thickenings were decided enough to give appreciable advantages to the

individuals distinguished by them--advantages in bearing the reactions

of the blows if not in inflicting the blows. But to conclude this

is, I think, to conclude against probability. Contrariwise, if we

assume that the thickening of the epidermis produced by habitual rough

usage is inheritable, the development of these structures presents no

difficulty. The points of impact would become indurated in wings used

for striking with unusual frequency. The callosities of surface thus

generated, rendering the parts less sensitive, would enable the bird in

which they arose to give, without injury to itself, more violent blows

and a greater number of them: so, in some cases, helping it to conquer

and multiply. Among its descendants, inheriting the modification and

the accompanying habit, the thickening would be further increased in

the same way: survival of the fittest tending ever to accelerate the

process. Presently the horny nodes so formed, hitherto defensive only

in their effects, would, by their prominence, become offensive--would

make the blows given more hurtful. And now natural selection, aiding

more actively, would mould the nodes into spurs: the individuals

in which the nodes were most pointed would be apt to survive and

propagate; and the pointedness generation after generation thus

increased, would end in the well-adapted shape we see.

But if in these cases the differentiations which fit particular parts

of the outer tissues to bear rough usage are caused mainly by the

direct balancing of external actions by internal reactions, then we

may suspect that the like is true of other modifications that occur

where special strains and abrasions have to be met. Possibly it is

true of sundry parts that are formed of hardened epidermis, such as

the nails, claws, hoofs, and hollow horns of Mammals; “all of which,”

says Prof. Huxley, “are constructed on essentially the same plan, being

diverticula of the whole integument, the outer layer of whose ecderon

has undergone horny metamorphosis.” Leaving open, however, the question

what tegumentary structures are due to direct equilibration, furthered

and controlled by indirect equilibration, it is tolerably clear that

direct equilibration has been one of the factors.

§ 294. Dermal structures of another class are developed mainly, if not

wholly, by the actions of external causes on species rather than on

individuals. These are the various kinds of clothing--hairs, feathers,

quills, scales, scutes. Though it is no longer thought as at one time

that all these various tegumentary structures are homologous with one

another, yet it is unquestionable that sundry of the more conspicuous

ones are. Those which are extremely unlike may be seen linked together

by a long series of graduated forms. A retrograde metamorphosis from

feathers to appendages that are almost scale-like, is well seen in

the coat of the Penguin. There is manifest a transition from the

bird-like covering to the fish-like covering--a transition so gradual

that no place can be found where an appreciable break occurs; and if

the scale-like appendages are not truly scales yet they exemplify

an extreme metamorphosis. Less striking, perhaps, but scarcely less

significant, are the modifications through which we pass from feathers

to hairs, on the surfaces of the Ostrich and the Cassowary. The skin

of the Porcupine shows us hairs and quills united by a series of

intermediate structures, differing from one another inappreciably. Even

more remarkable are certain other alliances of dermal structures. “It

may be taken as certain, I think,” says Prof. Huxley, “that the scales,

plates, and spines of all fishes are homologous organs; nor as less so

that the tegumentary spines of the Plagiostomes are homologous with

their teeth, and thence with the teeth of all vertebrata.”

Further details concerning these tegumentary structures are not needful

for present purposes, and are indeed but indirectly relevant to the

subject of physiological development. Here they are of interest to us

only by involving the general question--What physical influences have

brought them into existence? Still with a view to definite presentation

of the problem, it will be well to contemplate the mode of development

common to the most familiar of them.

Suppose a small pit to be formed on the previously flat skin; and

suppose that the growth and casting off of horny cells which goes on

over the skin in general, continues to go on at the usual rate over the

depressed surface of this pit. Clearly the quantity of horny matter

produced within this hollow, will be greater than that produced on a

level portion of the skin subtending an equal area of the animal’s

outside. Suppose such a pit to be deepened until it becomes a small

sac. If the exfoliation goes on as before, the result will be that the

horny matter, expelled, as it must be, through the mouth of the sac,

which now bears a small proportion to the internal surface of the sac,

will be large in quantity compared with that exfoliated from a portion

of the skin equal in area to the mouth of the sac: there will be a

conspicuous thrusting forth of horny matter. Suppose once more that

the sac, instead of remaining simple, has its bottom pushed up into

its interior, like the bottom of a wine-bottle--the introversion being

carried so far that the introverted part reaches nearly to the external

opening, and leaves scarcely any space between the introverted part and

the walls of the sac. It is easy to see that the exfoliation continuing

from the surface of the introverted part, as well as from the inside

of the sac generally, the horny matter cast off will form a double

layer; and will come out of the sac in the shape of a tube having

within its lower end the introverted part, as the core on which it is

moulded, and from the apex of which is cast off the substance filling,

less densely, its interior. The structure resulting will be what we

know as a hair. Manifestly by progressive enlargement of the sac, and

further complication of that introverted part on which the excreted

substance is moulded, the protruding growth may be rendered larger and

more involved, as we see it in quills and feathers. So that insensible

steps, thus indicated in principle, carry us from the exfoliation of

epidermis by a flat surface, to the exfoliation of it by a hollow

simple sac, an introverted sac, and a sac further complicated; each of

which produces its modified kind of tegumentary appendage.

But now, after contemplating this typical illustration, we return to

the general question. What are the agencies which have been operative

in developing these skin-structures? Indirect equilibration must have

worked almost alone in producing them. No direct incidence of forces

can have developed the enamelled armour of the _Lepidosteus_ or the

tesselated plates of the _Glyptodon_ and its modern allies. Survival of

the fittest must here and in multitudinous other cases be regarded as

the sole cause.

§ 295. Among many other differentiations of the outer tissues, the

most worthy to be noticed in the space that remains, are those by which

organs of sense are formed. We will begin with the simplest and most

closely-allied to the foregoing.

Every hair that is not too long or flexible to convey to its rooted end

a strain put upon its free end, is a rudimentary tactual organ; as may

be readily proved by touching one of those growing on the back of the

hand. If, then, a creature has certain hairs so placed that they are

habitually touched by the objects with which it deals, or amid which

it moves, an advantage is likely to accrue if these hairs are modified

in a way that enables them the better to transmit the impressions

derived. Such modified hairs we have in the _vibrissæ_, or, as they are

commonly called, the “whiskers” possessed by Cats and feline animals

generally, as well as by Seals and many Rodents. These hairs are long

enough to reach objects at considerable distances; they are so stiff

that forces applied to their free ends, cause movements of their

imbedded ends; and the sacs containing their imbedded ends being well

covered with nerve-fibres, these developed hairs serve as instruments

of exploration. By constant use of them the animal learns to judge of

the relative positions of objects past which, or towards which, it

is moving. When stealthily approaching prey or stealthily escaping

enemies, such aids to perception are obviously important: indeed their

importance has been proved by the diminished power of self-guidance in

the dark, that results from cutting them off. These, then, are dermal

appendages originally serving the purpose of clothing, but afterwards

differentiated into sense-organs.

That eyes are essentially dermal structures seems scarcely conceivable.

Yet an examination of their rudimentary types, and of their genesis

in creatures that have them well developed, shows us that they really

arise by successive modifications of the double layer composing the

integument. They make their first appearance among the simpler animals

as specks of pigment, covered by portions of epidermis slightly

convex and a little more transparent than that around it. Here their

fundamental community of structure with the skin is easy to trace; and

the formation of them by differentiation of it presents no difficulty.

Not so far in advance of these as much to obscure the relationship,

are the eyes which the Crustaceans possess. In every fishmonger’s shop

we may see that the eyes of a Lobster are carried on pedicles; and

when the Lobster casts its shell, the outer coat of each eye, being

continuous with the epidermis of its pedicle, is thrown off along with

the rest of the exo-skeleton. Beneath the transparent epidermic layer,

there exists a group of eyes of the kind which we see in an insect;

and these, according to a high authority, are inclosed in the dermal

system. Describing the arrangement of the parts, M. Milne Edwards

writes:--“But the most remarkable circumstance is, that the large

cavity within which the whole of these parallel columns, every one of

which is itself a perfect eye, are contained, is closed posteriorly by

a membrane, which appears to be neither more nor less than the middle

tegumentary membrane, pierced for the passage of the optic nerve; so

that the ocular chamber at large results from the separation at a point

of the two external layers of the general envelope.” Thus too is it,

in the main, even with the highly developed eyes of the _Vertebrata_.

“The three pairs of sensory organs appertaining to the higher senses,”

says Prof. Huxley--“the nasal sacs, the eyes, and the ears--arise as

simple cœcal involutions of the external integument of the head of

the embryo. That such is the case, so far as the olfactory sacs are

concerned, is obvious, and it is not difficult to observe that the

lens and the anterior chamber of the eye are produced in a perfectly

similar manner. It is not so easy to see that the labyrinth of the ear

arises in this way, as the sac resulting from the involution of the

integument is small, and remains open but a very short time. But I have

so frequently verified Huschke’s and Remak’s statement that it does so

arise, that I entertain no doubt whatever of the fact. The outer ends

of the olfactory sacs remain open, but those of the ocular and auditory

sacs rapidly close up, and shut off their contents from all direct

communication with the exterior.” That is to say, the eye considered as

an optical apparatus is produced by metamorphoses of the skin: the only

parts of it not thus produced, being the membranes lying between the

sclerotic and the vitreous humour, including those retinal structures

formed in them. All is tegumentary save that which has to appreciate

the impressions which the modified integument concentrates upon it.

Thus, as Prof. Huxley has somewhere pointed out, there is a

substantial parallelism between all the sensory organs in their modes

of development; as there is, too, between their modes of action.

A _vibrissa_ may be taken as their common type. Increased

impressibility by an external stimulus, requires an increased

peripheral expansion of the nervous system on which the stimulus may

fall; and this is secured by an introversion of the integument, forming

a sac on the walls of which a nerve may ramify. That the more extended

sensory area thus constituted may be acted upon, there requires some

apparatus conveying to it from without the appropriate stimulus; and

in the case of the _vibrissa_, this apparatus is the epidermic

growth which, under the form of a hair, protrudes from the sac. And

that the greatest sensitiveness may be obtained, the external action

must be exaggerated or multiplied by the apparatus which conveys it

to the recipient nerve; as, in the case of the _vibrissa_, it is

by the development of a hair into an elastic lever, that transforms

the slight force acting through considerable space on its exposed end,

into a greater force acting through a smaller space at its rooted end.

Similarly with the organs of the higher senses. In a rudimentary eye,

the slightly modified sense cell has but a rudimentary nerve to take

cognizance of the impression; and to concentrate the impression upon

it, there is nothing beyond a thickening of the epidermis into a

lens-shape. But the developed eye shows us a termination of the nerve

greatly expanded and divided to receive the external stimulus. It shows

us an introverted portion of the integument containing the apparatus

by which the external stimulus is conveyed to the recipient nerve. The

structure developed in this sac not only conveys the stimulus, but

also, like its homologue, concentrates it; and in the one case as in

the other, the structure which does this is an epidermic growth from

the bottom of the sac. Even with the ear it is the same. Again we have

an introverted portion of the integument, on the walls of which the

nerve is distributed in the primitive ear. The otolithes contained

in the sac thus formed, are bodies which are set in motion by the

vibrations of the surrounding water, and convey these vibrations in an

exaggerated form to the nerves. And though it is not alleged that these

otolithes are developed from the epidermic lining of the chamber, yet

as, if not so developed, they are concretions from the contents of an

epidermic sac, they must still be regarded as epidermic products.

Whether these differentiations are due wholly to indirect

equilibration, or whether direct equilibration has had a share in

working them, are questions that must be left open. Possibly a short

hair so placed on a mammal’s face as to be very often touched, may,

by conveying excitations to the nerves and vessels at its root, cause

extra growth of the bulb and its appendages, and so the development of

a _vibrissa_ may be furthered. Possibly, too, the light itself, to

which the tissues of some inferior animals are everywhere sensitive,

may aid in setting up certain of the modifications by which the nervous

parts of visual organs are formed: producing, as it must, the most

powerful effects at those points on the surface which the movements of

the animal expose to the greatest and most frequent contrasts of light

and shade; and propagating from those points currents of molecular

change through the organism. But it seems clear that the complexities

of the sensory organs are not thus explicable. They must have arisen by

the natural selection of favourable variations.

§ 296. A group of facts, serving to elucidate those put together in

the several foregoing sections, has to be added. I have reserved

this group to the last, partly because it is transitional--links the

differentiations of the literally outer tissues with those of the truly

inner tissues. Though physically internal, the mucous coat of the

alimentary canal has a _quasi_-externality from a physiological

point of view. As was pointed out in the last chapter, the skin and

the assimilating surface have this in common, that they come in direct

contact with matters not belonging to the organism; and we saw that

along with this community of relation to alien substances, there is a

certain community of structure and development. The like holds with the

linings of all internal cavities and canals that have external openings.

The transition from the literally outer tissues to those tissues

which are intermediate between them and the truly inner tissues,

is visible at all the orifices of the body; where skin and mucous

membrane are continuous, and the one passes insensibly into the

other. This visible continuity is associated not simply with a great

degree of morphological continuity, but also with a great degree of

physiological continuity. That is to say, these literally outer and

_quasi_-outer layers are capable of rapidly assuming one another’s

structures and functions when subject to one another’s conditions.

Mucous surfaces, normally kept covered, become skin-like if exposed

to the air; but resume more or less fully their normal characters

when restored to their normal positions. These are truths familiar to

pathologists. They continually meet with proofs that permanent eversion

of the mucous membrane, even where it is by prolapse of a part deeply

seated within the body, is followed by an adaptation eventually almost

complete: originally moist, tender to the touch, and irritated by the

air, the surface gradually becomes covered with a thick, dry cuticle;

and is then scarcely more sensitive than ordinary integument.

Whether this equilibration between new outer forces and reactive inner

forces, which is thus directly produced in individuals, is similarly

produced in races, must remain as a question not to be answered in a

positive way. On the one hand, we have the fact that among the higher

animals there are cases of _quasi_-outer tissues which are in

one species habitually ensheathed, while in another species they are

not ensheathed; and that these two tissues, though unquestionably

homologous, differ as much as skin and mucous membrane differ. On the

other hand, there are certain analogous changes of surface, as on the

abdomen of the Hermit-Crab, which give warrant to the supposition

that survival of the fittest is the chief agent in establishing such

differentiations; since the abdomen of a Hermit-Crab, bathed by water

within the shell it occupies, is not exposed to physical conditions

that directly tend to differentiate its surface from the surface of

the thorax. But though in cases like this last, we must assign the

result to the natural selection of variations arising incidentally; we

may, I think, legitimately assign the result to the immediate action

of changed conditions where, as in cases like the first, we see these

producing in the individual, effects of the kinds observed in the race.

However this may be, the force of the general argument remains the

same. In these exchanges of structure and function between the outer

and _quasi_-outer tissues, we get undeniable proof that they are

easily differentiable. And seeing this, we are enabled the more clearly

to see how there have, in course of time, arisen those extreme and

multitudinous differentiations of the outer tissues which have been

glanced at.

CHAPTER VIII.

DIFFERENTIATIONS AMONG THE INNER TISSUES OF ANIMALS.

§ 297. The change from the outside of the lips to their inside,

introduces us to a new series of interesting and instructive facts,

joining on to those with which the last chapter closed. They concern

the differentiations of those coats of the alimentary canal which, as

we have seen, are physiologically outer, though physically inner.

These coats are greatly modified at different parts; and their

modifications vary greatly in different animals. In the lower types,

where they compose a simple tube running from end to end of the body,

they are almost uniform in their histological characters; but on

ascending from these types, we find them presenting an increasing

variety of minute structures between their two ends. The argument

will be adequately enforced if we limit ourselves to the leading

modifications they display in some of the higher animals.

The successive parts of the alimentary canal are so placed with respect

to its contents, that the physical and chemical changes undergone by

its contents while passing from one end to the other, inevitably tend

to transform its originally homogeneous surface into a heterogeneous

surface. Clearly, the effect produced on the food at any part of

the canal by trituration, by adding a secretion, or by absorbing

its nutritive matters, implies the delivery of the food into the

next part of the canal in a state more or less unlike its previous

states--implies that the surface with which it now comes in contact

is differently affected by it from the preceding surfaces--implies,

that is, a differentiating action. To use concrete language;--food

that is broken down in the mouth acts on the œsophagus and stomach

in a way unlike that which it would have done had it been swallowed

whole; the masticated food, to which certain solvents or ferments

are added, becomes to the intestine a different substance from that

which it must have otherwise been; and the altered food, resolved

by these additions into its proximate principles, cannot have those

proximate principles absorbed in the next part of the intestine,

without the remoter parts being affected as they would not have been

in the absence of absorption. It is true that in developed alimentary

canals, such as the reasoning here tacitly assumes, these marked

successive differentiations of the food are themselves the results

of pre-established differentiations in the successive parts of the

canal. But it is also true that actions and reactions like those

here so definitely marked, must go on indefinitely in an undeveloped

alimentary canal. If the food is changed at all in the course of its

transit, which it must be if the creature is to live by it, then it

cannot but act dissimilarly on the successive tracts of the alimentary

canal, and cannot but be dissimilarly reacted on by them. Inevitably,

therefore, the uniformity of the surface must lapse into greater or

less multiformity: the differentiation of each part tending ever to

initiate differentiations of other parts.

Not, indeed, that the implied process of direct equilibration can

be regarded as the sole process. Indirect equilibration aids; and,

doubtless, there are some of the modifications which only indirect

equilibration can accomplish. But we have here one unquestionable

cause--a cause that is known to work in individuals, changes of the

kind alleged. Where, for instance, cancerous disease of the œsophagus

so narrows the passage into the stomach as to prevent easy descent of

the food, the œsophagus above the obstruction becomes enlarged into a

kind of pouch; and the inner surface of this pouch begins to secrete

juices that produce in the food a kind of rude digestion. Again,

stricture of the intestine, when it arises gradually, is followed by

hypertrophy of the muscular coat of the intestine above the constricted

part: the ordinary peristaltic movements being insufficient to force

the food forwards, and the lodged food serving as a constant stimulus

to contraction, the muscular fibres, habitually more exercised,

become more bulky. The deduction from general principles being thus

inductively enforced, we cannot, I think, resist the conclusion that

the direct actions and reactions between the food and the alimentary

canal have been largely instrumental in establishing the contrasts

among its parts. And we shall hold this view with the more confidence

on observing how satisfactorily, in pursuance of it, we are enabled to

explain one of the most striking of these differentiations, which we

will take as a type of the class.

The gizzard of a bird is an expanded portion of the alimentary canal,

specially fitted to give the food that trituration which the toothless

mouth of a bird cannot give. Besides having a greatly-developed

muscular coat, this grinding-chamber is lined with a thick, hard

cuticle, capable of bearing the friction of the pebbles swallowed to

serve as grindstones. This differentiation of the mucous coat into a

ridged and tubercled layer of horny matter--a differentiation which,

in the analogous organs of certain _Mollusca_, is carried to the

extent of producing from this membrane cartilaginous plates, and even

teeth--varies in birds of different kinds, according to their food.

It is moderate in birds that feed on flesh and fish, and extreme in

granivorous birds and others that live on hard substances. How does

this immense modification of the alimentary canal originate? In the

stomach of a mammal, the macerating and solvent actions are united

with that triturating action which finishes what the teeth have mainly

done; but in the bird, unable to masticate, these internal functions

are specialized, and while the crop is the macerating chamber, the

gizzard becomes a chamber adapted to triturate more effectually. This

adaptation requires simply an exaggeration of certain structures

and actions which characterize stomachs in general, and, in a less

degree, alimentary canals throughout their whole lengths. The massive

muscles of the gizzard are simply extreme developments of the muscular

tunic, which is already considerably developed over the stomach, and

incloses also the œsophagus and the intestine. The indurated lining of

the gizzard, thickened into horny buttons at the places of severest

pressure, is nothing more than a greatly strengthened and modified

epithelium. And the grinding action of the gizzard is but a specialized

form of that rhythmical contraction by which an ordinary stomach kneads

the contained food, and which in the œsophagus effects the act of

swallowing, while in the intestine it becomes the peristaltic motion.

Allied as the gizzard thus clearly is in structure and action to the

stomach and alimentary canal in general; and capable of being gradually

differentiated from a stomach where a growing habit of swallowing

food unmasticated entails more trituration to be performed before

the food passes the pylorus; the question is--Does this change of

structure arise by direct adaptation? There is warrant for the belief

that it does. Besides such collateral evidence as that mucous membrane

becomes horny on the toothless gums of old people, when subject to

continual rough usage, and that the muscular coat of the intestine

thickens where unusual activity is demanded of it, we have the direct

evidence of experiment. Hunter habituated a sea-gull to feed on grain,

and found that the lining of its gizzard became hardened, while the

gizzard-muscles doubled in thickness. A like change in the diet of a

kite was followed by like results. Clearly, if differentiations so

produced in the individuals of a race under changed habits, are in any

degree inheritable, a structure like a gizzard will originate through

the direct actions and reactions between the food and the alimentary

canal.

Another case--a very interesting one, somewhat allied to this--is

presented by the ruminating animals. Here several dilatations of

the alimentary canal precede the true stomach; and in them large

quantities of unmasticated food are stored, to be afterwards returned

to the mouth and masticated at leisure. What conditions have made

this specialization advantageous? and by what process has it been

established? To both these questions the facts indicate answers

which are not unsatisfactory. [Creatures that obtain their food very

irregularly--now having more than they can consume, and now being for

long periods without any--must, in the first place, be apt, when very

hungry, to eat to the extreme limits of their capacities; and must,

in the second place, profit by peculiarities which enable them to

compensate themselves for long fasts, past and future. A perch which,

when its stomach is full of young frogs, goes on filling its œsophagus

also; or a trout which, rising to the fisherman’s fly, proves when

taken off the hook to be full of worms and insect-larvæ up to the very

mouth, gains by its ability to take in such unusual supplies of food

when it meets with them--obviously thrives better than it would do

could it never eat more than a stomachful. That this ability to feed

greatly in excess of immediate requirement, is one that varies in

individuals of the same race, we see in the marked contrast between

our own powers in this respect, and the powers of uncivilized men;

whose fasting and gorging are to us so astonishing. Carrying with us

these considerations, we shall not be surprised at finding dilatations

of the œsophagus in vultures and eagles, which get their prey at long

intervals in large masses; and we may naturally look for them, too, in

birds like pigeons, which, coming in flocks upon occasional supplies

of grain, individually profit by devouring the greatest quantity in

a given time. Now where the trituration of the food is, as in these

cases, carried on in a lower part of the alimentary canal, nothing

further is required than the storing-chamber; but for a mammal,

having its grinding apparatus in its mouth, to gain by the habit of

hurriedly swallowing unmasticated food, it must also have the habit of

regurgitating the food for subsequent mastication. This correlation of

habits with their answering structures, may, as we shall see, arise in

a very simple way. The starting point of the explanation is a familiar

fact--the fact that indigestion, often resulting from excess of food,

is apt to cause that reversed peristaltic action known as vomiting.

From this we pass to the fact, also within the experience of most

persons, that during slight indigestion the stomach sometimes quietly

regurgitates a small part of its contents as far as the back of the

mouth--giving an unpleasant acquaintance with the taste of the gastric

juices. Exceptional facts of the same class help the argument a step

further. “There are certain individuals who are capable of returning,

at will, a greater or smaller portion of the contents of the digesting

stomach into the cavity of the mouth.... In some of these cases, the

expulsion of the food has required a violent effort. In the majority

it has been easily evoked or suppressed. While in others, it has been

almost uncontrollable; or its non-occurrence at the habitual time

has been followed by a painful feeling of fulness, or by the act of

vomiting.” Here we have a certain physiological action, occasionally

happening in most persons and in some developed into a habit more or

less pronounced: indigestion being the habitual antecedent. Suppose,

then, that gregarious animals, living on innutritive food such as

grass, are subject to a like physiological action, and are capable of

like variations in the degree of it. What will naturally happen? They

wander in herds, now over places where food is scarce and now coming

to places where it is abundant. Some masticate their food completely

before swallowing it, while some masticate it incompletely. If an

oasis, presently bared by their grazing, has not supplied to the whole

herd a full meal, then the individuals which masticate completely

will have had less than those which masticate incompletely--will

not have had enough. Those which masticate incompletely and distend

their stomachs with food difficult to digest, will be liable to these

regurgitations; but if they re-masticate what is thus returned to

the mouth (and we know that animals often eat again what they have

vomited), then the extra quantity of food taken, eventually made

digestible, will yield them more nourishment than is obtained by those

which masticate completely at first. The habit initiated in this

natural way, and aiding survival when food is scarce, will be apt to

cause modifications of the alimentary canal. We know that dilatations

of canals readily arise under habitual distensions. We know that canals

habitually distended become gradually more tolerant of the contained

masses that at first irritated them. And we know that there commonly

take place adaptive modifications of their surfaces. Hence if a habit

of this kind and the structural changes resulting from it, are in

any degree inheritable, it is clear that, increasing in successive

generations, both immediately by the cumulative effect of repetitions

and mediately by survival of the individuals in which they are most

decided, they may go on until they end in the peculiarities which

Ruminants display.

§ 298. There are structures belonging to the same group which cannot,

however, be accounted for in this way. They are the organs that

secrete special products facilitating digestion--the liver, pancreas,

and various smaller glands. All these appendages of the alimentary

canal, large and independent as some of them seem, really arise by

differentiations from its coats. The primordial liver consists of

nothing more than bile-cells scattered along a tract of the intestinal

surface. Accumulation of these bile-cells is accompanied by increased

growth of the surface which bears them--a growth which at first takes

the form of a _cul-de-sac_, having an outside that projects from the

intestine into the peri-visceral cavity. As the mass of bile-cells

becomes greater, there arise secondary lateral cavities opening into

the primary one, and through it into the intestine; until, eventually,

these cavities with their coatings of bile-cells, become ramifying

ducts distributed through the solid mass we know as a liver. How is

this differentiation caused?

Before attempting any answer to this question, it is requisite to

inquire the nature of bile. Is that which the liver throws into

the intestines a waste product of the organic actions? or is it a

secretion aiding digestion? or is it a mixture of these? Modern

investigations imply that it is most likely the last. The liver

is found to have a compound function. Bernard has proved to the

satisfaction of physiologists, that there goes on in it a formation of

glycogen--a substance which is transformed into sugar before it leaves

the liver and is afterwards carried away by the blood to eventually

disappear in the active organs, chiefly the muscles. It is also shown,

experimentally, that there are generated in the liver certain biliary

acids; and by the aid either of these or of some other compounds, it is

clear that bile renders certain materials more absorbable. Its effect

on fat is demonstrable out of the body; and the greatly diminished

absorption of fat from the food when the discharge of bile into the

intestine is prevented, is probably one of the causes of that pining

away which results. But while recognizing the fact that the bile

consists in part of a solvent, or solvents, aiding digestion, there

is abundant evidence that one element of it is an effete product;

and probably this is the primary element. The yellow-green substance

called biliverdine in herbivora and bilirubin in man and carnivora,

which gives its colour to bile, is a product the greater part of which

is normally cast out from the system continually, as is shown by the

contrast between the normal and abnormal colours of fæcal matters, and

as is still more strikingly shown by the effects on the system when

there is a stoppage of the excretion, and an attack of jaundice. Hence

we are warranted in classing biliverdine as a waste product, and we

may fairly infer that the excretion of it is the original function of

the liver.

One further preliminary is requisite. We must for a moment return to

those physico-chemical data set down in the first chapter of this work

(§§ 7–8). We there saw that the complex and large-atomed colloids which

mainly compose living organic matter, have extremely little molecular

mobility; and, consequently, extremely little power of diffusing

themselves. Whereas we saw not only that those absorbed matters,

gaseous and liquid, which further the decomposition of living organic

matter, have very high diffusibilities, but also that the products

of the decomposition are much more diffusible than the components of

living organic matter. And we saw that, as a consequence of this, the

tissues give ready entrance to the substances which decompose them,

and ready exit to the substances into which they are decomposed. Hence

it follows that, under its initial form, uncomplicated by nervous and

other agencies, the escape of effete matters from the organism, is a

physical action parallel to that which goes on among mixed colloids

and crystalloids that are dead or even inorganic. Excretion is a

specialized form of this spontaneous action; and we have to inquire how

the specialization arises.

Two causes conspire to establish it. The first is that these products

of decomposition are diffusible in widely different degrees. While

the carbonic acid and water permeate the tissues with ease in all

directions, and escape more or less from the exposed surfaces, urea,

and other waste substances incapable of being vaporized, cannot escape

thus readily. The second is that the different parts of the body, being

subject to different physical conditions, are from the outset sure

severally to favour the exit of these various products of decomposition

in various degrees. How these causes must have co-operated in

localizing the excretions, we shall see on remembering how they now

co-operate in localizing the separation of morbid materials. The

characteristic substances of gout and rheumatism have their habitual

places of deposit. Tuberculous matter, though it may be present in

various organs, gravitates towards some much more than towards others.

Certain products of disease are habitually got rid of by the skin,

instead of collecting internally. Mostly, these have special parts of

the skin which they affect rather than the rest; and there are those

which, by breaking out symmetrically on the two sides of the body,

show how definitely the places of their excretion are determined by

certain favouring conditions, which corresponding parts may be presumed

to furnish in equal degrees. Further, it is to be observed of these

morbid substances circulating in the blood, that having once commenced

segregating at particular places, they tend to continue segregating

at those places. Assuming, then, as we may fairly do, that this

localization of excretion, which we see continually commencing afresh

with morbid matters, has always gone on with the matters produced by

the waste of the tissues, let us take a further step, and ask how

localizations become fixed. Other things equal, that which from its

physical conditions is a place of least resistance to the exit of

an effete product, will tend to become established as the place of

excretion; since the rapid exit of an effete product will profit the

organism. Other things equal, a place at which the excreted matter

produces least detrimental effect will become the established place. If

at any point the excreted matter produces a beneficial effect, then,

other things equal, survival of the fittest will determine it to this

point. And if facility of escape anywhere goes along with utilization

of the escaping substance, then, other things equal, the excretion will

be there localized still more decisively by survival of the fittest.

Such being the conditions of the problem, let us ask what will happen

with the lining membrane of the alimentary canal. This, physiologically

considered, is an external surface; and matters thrown off from it

make their way out of the body. It is also a surface along which is

moving the food to be digested. Now, among the various waste products

continually escaping from the living tissues, some of the more complex

ones, not very stable in composition, are likely, if added to the food,

to set up changes in it. Such changes may either aid or hinder the

preparation of the food for absorption. If an effete matter, making its

exit through the wall of the intestine, hinders the digestive process,

the enfeeblement and disappearance of individuals in which this

happens, will prevent the intestine from becoming the established place

for its exit. While if it aids the digestive process, the intestine

will, for converse reasons, become more and more the place to which

its exit is limited. Equally manifest is it that if there is one part

of this alimentary canal at which, more than at any other part, the

favourable effect results, this will become the place of excretion.

Thus, then, reverting to the case in question, we may understand how a

product to be cast out, such as biliverdine, if it either directly or

indirectly serves a useful purpose, when poured into a particular part

of the intestine, may lead to the formation of a patch of excreting

cells on its wall; and once this place of excretion having been

established, the development of a liver is simply a question of time

and natural selection.

§ 299. A differentiation of another order occurring in the alimentary

canal, is that by which a part of it is developed into a lateral

chamber or chambers, through which carbonic acid exhales and oxygen

is absorbed. Comparative anatomy and embryology unite in showing

that a lung is formed, just as a liver or other appendage of the

alimentary canal is formed, by the growth of a hollow bud into the

peri-visceral cavity, or space between the alimentary canal and the

wall of the body. The interior of this bud is simply a _cul-de-sac_ of

the alimentary canal, with the mucous lining of which its own mucous

lining is continuous. And the development of this _cul-de-sac_ into

an air-chamber, simple or compound, is merely a great extension of

area in the internal surface of the _cul-de-sac_, along with that

specialization which fits it for excreting and absorbing substances

different from those which other parts of the mucous surface excrete

and absorb. These lateral air-chambers, universal among the higher

_Vertebrata_ and very general among the lower, and everywhere attached

to the alimentary canal between the mouth and the stomach, have not

in all cases the respiratory function. In most fishes that have them

they are what we know as swim-bladders. In some fishes the cavities of

these swim-bladders are completely shut off from the alimentary canal:

nevertheless showing, by the communications which they have with it

during the embryonic stages, that they are originally _diverticula_

from it. In other fishes there is a permanent _ductus pneumaticus_,

uniting the cavity of the swim-bladder with that of the gullet: the

function, however, being still not respiratory in an appreciable

degree, if at all. But in certain still extant representatives of the

sauroid fishes, as the _Lepidosteus_, the air-bladder is “divided into

two sacs that possess a cellular structure,” and “the trachea which

proceeds from it opens high up in the throat, and is surrounded with

a glottis.” In the _Amphibia_ the corresponding organs are chambers

over the surfaces of which there are saccular depressions, indicating a

transition towards the air-cells characterizing lungs; and accompanying

this advance we see, as in the common _Triton_, the habit of coming up

to the surface and taking down a fresh supply of air in place of that

discharged.

How are the internal air-chambers, respiratory or nonrespiratory,

developed? Upwards from the amphibian stage, in which they are

partially refilled at long intervals, there is no difficulty in

understanding how, by infinitesimal steps, they pass into complex

and ever-moving lungs. But how is the differentiation that produces

them initiated? How comes a portion of the internal surface to be

specialized for converse with a medium to which it is not naturally

exposed? The problem appears a difficult one; but there is a not

unsatisfactory solution of it.

When many gold-fish are kept in a small aquarium, as with thoughtless

cruelty they frequently are, they swim close to the surface, so as to

breathe that water which is from instant to instant absorbing fresh

oxygen. In doing this they often put their mouths partly above the

surface, so that in closing them they take in bubbles of air; and

sometimes they may be seen to continue doing this--the relief due to

the slight extra aëration of blood so secured, being the stimulus

to continue. Air thus taken in may be detained. If a fish that has

taken in a bubble turns its head downwards, the bubble will ascend

to the back of its mouth, and there lodge; and coming within reach

of the contractions of the œsophagus, it may be swallowed. If, then,

among fish thus naturally led upon occasion to take in air-bubbles,

there are any having slight differences in the alimentary canal that

facilitate lodgment of the air, or slight nervous differences such as

in human beings cause an accidental action to become “a trick,” it

must happen that if an advantage accrues from the habitual detention

of air-bubbles, those individuals most apt to detain them will,

other things equal, be more likely than the rest to survive; and by

the survival of descendants inheriting their peculiarities in the

greatest degrees, and increasing them, an established structure and an

established habit may arise. And that they do in some way arise we have

proof. The common Loach swallows air, which it afterwards discharges

loaded with carbonic acid.

From air thus swallowed the advantages that may be derived are of two

kinds. In the first place, the fish is made specifically lighter, and

the muscular effort needed to keep it from sinking is diminished--or,

indeed, if the bubble is of the right size, is altogether saved.

The contrast between the movements of a Goby, which, after swimming

up towards the surface, falls rapidly to the bottom on ceasing its

exertions, and the movements of a Trout, which remains suspended just

balancing itself by slight undulations of its fins, shows how great

an economy results from an internal float, to fishes which seek their

food in mid-water or at the surface. Hence the habit of swallowing

air having been initiated in the way described, we see why natural

selection will, in certain fishes, aid modifications of the alimentary

canal favouring its lodgment--modifications constituting air-sacs.

In the second place, while from air thus lodged in air-sacs thus

developed, the advantage will be that of flotation only if the air

is infrequently changed or never changed, the advantage will be that

of supplementary respiration if the air-sacs are from time to time

partially emptied and refilled. The requirements of the animal will

determine which of the two functions predominates. Let us glance at the

different sets of conditions under which these divergent modifications

may be expected to arise.

The respiratory development is not likely to take place in fishes

that inhabit seas or rivers in which the supply of aërated water

never fails: there is no obvious reason why the established branchial

respiration should be replaced by a pulmonic respiration. Indeed, if

a fish’s branchial respiration is adequate to its needs, a loss would

result from the effort of coming to the surface for air; especially

during those first stages of pulmonic development when the extra

aëration achieved was but small. Hence in fishes so circumstanced,

the air-chambers arising in the way described would naturally become

specialized mainly or wholly into floats. Their contained air being

infrequently changed, no advantage would arise from the development

of vascular plexuses over their surfaces; nothing would be gained

by keeping open the communication between them and the alimentary

canal; and there might thus eventually result closed chambers the

gaseous contents of which, instead of being obtained from without,

were secreted from their walls, as gases often are from mucous

membranes. Contrariwise, aquatic vertebrates in which the swallowing of

air-bubbles, becoming habitual, had led to the formation of sacs that

lodged the bubbles; and which continued to inhabit waters not always

supplying them with sufficient oxygen, might be expected to have the

sacs further developed, and the practice of changing the contained

air made regular, if either of two advantages resulted--either the

advantage of being able to live in old habitats that had become

untenable without this modification, or the advantage of being able to

occupy new habitats. Now it is just where these advantages are gained

that we see the pulmonic respiration coming in aid of the branchial

respiration, and in various degrees replacing it. Shallow waters are

liable to three changes which conspire to make this supplementary

respiration beneficial. The summer’s sun heats them, and raising the

temperatures of the animals they contain, accelerates the circulation

in these animals, exalts their functional activities, increases the

production of carbonic acid, and thus makes aëration of the blood more

needful than usual. Meanwhile the heated water, instead of yielding

to the highly carbonized blood brought to the branchiæ the usual

quantity of oxygen, yields less than usual; for as the heat of the

water increases, the quantity of air it contains diminishes. And this

greater demand for oxygen joined with smaller supply, pushed to an

extreme where the water is nearly all evaporated, is at last still

more intensely felt in consequence of the excess of carbonic acid

discharged by the numerous creatures congregated in the muddy puddles

that remain. Here, then, it is, that the habit of taking in air-bubbles

is likely to become established, and the organs for utilizing them

developed; and here it is, accordingly, that we find all stages of the

transition to aërial respiration. The Loach before-mentioned, which

swallows air, frequents small waters liable to be considerably warmed.

The _Amphipnous Cuchia_, an anomalous eel-shaped fish, which has

vascular air-sacs opening out at the back of the mouth, “is generally

found lurking in holes and crevices, on the muddy banks of marshes or

slow-moving rivers”; and though its air-sacs are not morphological

equivalents of those above described, yet they equally well illustrate

the relation between such organs and the environing condition. Still

more significant is the fact that the _Lepidosiren_, or “mudfish” as it

is called from its habits, though it is a true fish nevertheless has

lungs. But it is among the _Amphibia_ that we see most conspicuously

this relation between the development of air-breathing organs, and

the peculiarities of the habitats. Pools, more or less dissipated

annually, and so rendered uninhabitable by most fishes, are very

generally peopled by these transitional types. Just as we see, too,

that in various climates and in various kinds of shallow waters, the

supplementary aërial respiration is needful in different degrees; so

do we find among the _Amphibia_ many stages in the substitution of the

one respiration for the other. The facts, then, are such as give to the

hypothesis a _vraisemblance_ greater than could have been expected.

The relative effects of direct and indirect equilibration in

establishing this further heterogeneity, must, as in many other

cases, remain undecided. The habit of taking in bubbles is scarcely

interpretable as a result of spontaneous variation: we must regard it

as arising accidentally during the effort to obtain the most aërated

water; as being persevered in because of the relief obtained; and as

growing by repetition into a tendency bequeathed to offspring, and

by them, or some of them, increased and transmitted. The formation

of the first slight modifications of the alimentary canal favouring

the lodgment of bubbles, is not to be thus explained. Some favourable

variation in the shape of the passage must here have been the initial

step. But the gradual increase of this structural modification by the

survival of individuals in which it is carried furthest, will, I think,

be all along aided by immediate adaptation. The part of the alimentary

canal previously kept from the air, but now habitually in contact with

the air, must be in some degree modified by the action of the air;

and the directly-produced modification, increasing in the individual

and in successive individuals, cannot cease until there is a complete

balance between the actions of the changed agency and the changed

tissue.

§ 300. We come now to differentiations among the truly inner

tissues--the tissues which have direct converse neither with the

environment nor with the foreign substances taken into the organism

from the environment. These, speaking broadly, are the tissues

which lie between the double layer forming the integument with its

appendages, and the double layer forming the alimentary canal with

its _diverticula_. We will take first the differentiation which

produces the vascular system.

Certain forces producing and aiding distribution of liquids in animals,

come into play before any vascular system exists; and continue to

further circulation after the development of a vascular system. The

first of these is osmotic exchange, acting locally and having an

indirect general action; the second is local variation of pressure,

which movement of the body throws on the tissues and their contained

liquids. A few words are needed in elucidation of each. If in any

creature, however simple, different changes are going on in parts

that are differently conditioned--if, as in a _Hydra_, one surface

is exposed to the surrounding medium while the other surface is

exposed to dissolved food; then between the unlike liquids which the

dissimilarly-placed parts contain, osmotic currents must arise; and

a movement of liquid through the intermediate tissue must go on as

long as an unlikeness between the liquids is kept up. This primary

cause of re-distribution remains one of the causes of re-distribution

in every more-developed organism: the passage of matters into and

out of the capillaries is everywhere thus set up. And obviously in

producing these local currents, osmose must also indirectly produce

general currents, or aid them if otherwise produced. In the absence

of a pumping organ, this force is probably an important aid to that

movement of the nutritive liquids which the functions set up. How

the second cause--the changes of internal pressure which an animal’s

movements produce--furthers circulation, will be sufficiently manifest.

That parts which are bent or strained necessarily have their contained

vessels squeezed, has been shown (§ 281); and whether the bend or

strain is caused, as in a plant, by an external force, or, as usually

in an animal, by an internal force, there must be a thrusting of

liquids towards places of least resistance--commonly places of greatest

consumption. This which in animals without hearts is a main agent of

circulation, continues to further it very considerably even among the

highest animals. In these the effect becomes as it were systematized.

The valves in the veins necessitate perpetual propulsions towards the

heart.

Even in such simple types as the _Hydrozoa_, cavities in the tissues

faintly indicate a structure which facilitates the transfer of

nutritive matters. These cavities become reservoirs filled with the

plasma that slowly oozes through the substance of the body; and every

movement of the animal, accompanied as it must be by changed pressures

and tensions on these reservoirs, tends here to fill them and there to

squeeze out their contents in that or the other direction--possibly

aiding to produce, by union of several cavities, those lacunæ or

irregular canals which the body in some cases presents.

Irregular canals of this kind, not lined with any membranes but being

simply cavities running through the flesh, mainly constitute the

vascular system in _Polyzoa_ and _Brachiopoda_ and some _Mollusca_.

Though the central parts of a vascular system are rudely developed, yet

its peripheral parts consist of sinuses permeating the tissues. The

higher orders of _Mollusca_ have a more-developed system of vessels or

arteries, which run into the substance of the body and end in lacunæ or

simple fissures. This ending in lacunæ takes place at various distances

from the vascular centre. In some genera the arterial structure is

carried to the periphery of the blood-system, while in others it stops

short midway. Throughout most orders of the _Mollusca_ the back

current of blood continues to be carried by channels of the original

kind: there are no true veins, but the blood having been delivered

into the tissues, finds its way back to the peri-visceral cavity

through inosculating sinuses. Among the Cephalopods, however, the

afferent blood-canals, as well as the efferent ones, acquire distinct

walls. On putting together these facts, we may conceive pretty clearly

the stages of vascular development. From the original reservoir of

nutritive liquid between the alimentary canal and the wall of the body,

a portion partially shut off becomes a contractile vessel; and by its

actions there is produced a more rapid transfer of the nutritive liquid

than was originally produced by the motions of the animal. Clearly,

the extension of this contractile tube and the development from it

of branches running hither and thither into the tissues, must, by

defining the channels of blood throughout a part of its course, render

its distribution more regular and active. As fast as this centrifugal

growth advances, so fast are the efferent currents of blood, prevented

from escaping laterally, obliged to move from the centre towards

the circumference; and so fast also does the less developed set of

channels become, of necessity, occupied by afferent currents. When, by

a parallel increase of definiteness, the lacunæ and irregular sinuses

through which the afferent currents pass, become transformed into

veins, the accompanying disappearance of all stagnant or slow-moving

collections of blood, implies a further improvement in the circulation.

By what agency is effected this differentiation of a definite vascular

system? No sufficient reply is obvious. The genesis of the primordial

heart is not comprehensible as a result of direct equilibration, and we

cannot readily see our way to it as a result of indirect equilibration;

for it is difficult to imagine what favourable variation natural

selection could have seized hold of to produce such a structure. A

contractile tube that aided the distribution of nutritive liquid,

having been once established, survival of the fittest would suffice

for its gradual extension and its successive modifications. But what

were the early stages of the contractile tube, while it was yet not

sufficiently formed to help circulation, and while it must nevertheless

have had some advantage without which no selective process could go

on? The question seems insoluble. To another part of the question,

however, an answer may be ventured. If we ask the origin of these

ramifying channels which, first appearing as simple lacunæ, eventually

become vessels having definite walls, a reply admitting of considerable

justification, is, that the currents of nutritive liquid forced and

drawn hither and thither through the tissues, themselves initiate these

channels. We know that streams running over and through solid and

quasi-solid inorganic matter, tend to excavate definite courses. We saw

reason for concluding that the development of sap-channels in plants

conforms to this general principle. May we not then suspect that the

nutritive liquid contained in the tissue of a simple animal, made to

ooze now in this direction and now in that by the changes of pressure

which the animal’s movements cause, comes to have certain lines along

which it is thrust backwards and forwards more than along other lines;

and must by repeated passings make these more and more permeable until

they become lacunæ? Such actions will inevitably go on; and such

actions appear competent to produce some, at least, of the observed

effects. The leading facts which indicate that this is a part-cause of

vascular development are these.

Growths normally recurring in certain places at certain intervals,

are accompanied by local formations of blood-vessels. The periodic

maturation of ova among the _Mammalia_ supplies an instance. Through

the stroma of an ovarium are distributed innumerable minute vesicles,

which, in their early stages, are microscopic. Of these, severally

contained in their minute ovi-sacs, any one may develop: the

determining cause being probably some slight excess of nutrition. When

the development is becoming rapid, the capillaries of the neighbouring

stroma increase and form a plexus on the walls of the ovi-sac. Now

since there is no typical distribution of the developing ova; and since

the increase of an ovum to a certain size precedes the increase of

vascularity round it; we can scarcely help concluding that the setting

up of currents towards the point of growth determines the formation of

the blood-vessels. It may be that having once commenced, this local

vascular structure completes itself in a typical manner; but it seems

clear that this greater development of blood-vessels around the growing

ovum is initiated by the draught towards it. Abnormal growths show

still better this relation of cause and effect. The false membranes

sometimes found in the bronchial tubes in inflammatory diseases, may

perhaps fairly be held abnormal in but a partial sense: it may be said

that their vascular systems are formed after the type of the membranes

to which they are akin. But this can scarcely be said of the morbid

growths classed as malignant. The blood-vessels in an encephaloid

cancer, are led to enlarge and ramify, often to an immense extent, by

the unfolding of the morbid mass to which they carry blood. Alien as

is the structure as a whole to the type of the organism; and alien in

great measure as is its tissue to the tissue on which it is seated;

it nevertheless happens that the growth of the alien tissue and

accompanying abstraction of materials from the blood-vessels, determine

a corresponding growth of these blood-vessels. Unless, then, we say

that there is a providentially-created type of vascular structure for

each kind of morbid growth (and even this would not much help us, since

the vascular structure has no constancy within the limits of each

kind), we are compelled to admit that in some way or other the currents

of blood are here directly instrumental in forming their own channels.

One more piece of evidence, before cited as exemplifying adaptation (§

67), may be called to mind. When any main channel for blood, leading

to or from a certain part of the body, has been rendered impervious,

others among the channels leading to or from this same part, enlarge to

the extent requisite for fulfilling the extra function that falls upon

them: the enlargement being caused, as we must infer, by the increase

of the currents carried.

Here, then, are facts warranting inductively the deduction above drawn.

It is true that we are left in the dark respecting the complexities

of the process. How the channels for blood come to have limiting

membranes, and many of them muscular coats, the hypothesis does not

help us to say. But the evidence assigned goes far to warrant the

belief that vascular development is initiated by direct equilibration;

though indirect equilibration may have had the larger share in

establishing the structures which distinguish finished vascular systems.

§ 301. Of the inner tissues which remain let us next take bone. In what

manner is differentiated this dense substance serving in most cases for

internal support?

When considering the vertebrate skeleton under its morphological aspect

(§ 256), it was pointed out that the formation of dense tissues,

internal as well as external, is, in some cases at least, brought

about by the mechanical forces to be resisted. Through what process

it is brought about we could not then stay to inquire: this question

being not morphological but physiological. Answers to some kindred

questions have since been attempted. Certain actions to which the

internal dense tissues of plants may be ascribed, have been indicated;

and more recently, analogous actions have been assigned as causes of

some external dense tissues of animals. We have now to ask whether

actions of the same nature have produced these internal dense tissues

of animals.

The problem is an involved one. Bones have more than one stage. They

are membranous or cartilaginous before they become osseous; and their

successive component substances so far differ that the effects

of mechanical actions upon them differ. And having to deal with

transitional states in which bone is formed of mixed tissues, having

unlike physical properties and unlike minute structures, the effects of

strains become too complicated to follow with precision. Anything in

the way of interpretation must therefore be regarded as tentative. If

analysis and comparison show that the phenomena are not inconsistent

with the hypothesis of mechanical genesis, it is as much as can be

expected. Let us first observe more nearly the mechanical conditions to

which bones are subject.

The endo-skeleton of a mammal with the muscles and ligaments holding

it together, may be rudely compared to a structure built up of struts

and ties; of which, speaking generally, the struts bear the pressures

and the ties bear the tensions. The framework of an ordinary iron

roof will give an idea of the functions of these two elements, and of

the mechanical characters required by them. Such a framework consists

partly of pieces which have each to bear a thrust in the direction of

its length, and partly of pieces which have each to bear a pull in the

direction of its length; and these struts and ties are differently

formed to adapt them to these different strains. Further, it should

be remarked that though the rigidity of the framework depends on the

ties which are flexible, as much as on the struts which are stiff, yet

the ties help to give the rigidity simply by so holding the struts

in position that they cannot escape from the thrusts which fall on

them. Now the like relation holds with a difference among the bones

and muscles: the difference being that here the ties admit of being

lengthened or shortened and the struts of being moved about upon their

joints. The mechanical relations are not altered by this, however.

The actions are of essentially the same kind in an animal that is

standing, or keeping itself in a strained attitude, as in one that is

changing its attitude--the same in so far that we have in each a set

of flexible parts that are pulling and a set of rigid parts that are

resisting. It needs but to remember the sudden collapse and fall which

take place when the muscles are paralyzed, or to remember the inability

of a bare skeleton to support itself, to see that the struts without

the ties cannot suffice. And we have but to think of the formless mass

into which a man would sink when deprived of his bones, to see that

the ties without the struts cannot suffice. To trace the way in which

a particular bone has its particular thrust thrown upon it, may not

always be practicable. Though it is easy to perceive how a flexor or

extensor of the arm causes by its tension a reactive pressure along the

line of the humerus, and is enabled to produce its effect only by the

rigidity of the humerus; yet it is not so easy to perceive how such

bones as those of a horse’s pelvis are similarly acted upon. Still, as

the weight of the hind quarters has to be transferred from the back to

the feet, and must be so transferred through the bones, it is manifest

that though these bones form a very crooked line, the weight must

produce a pressure along the axis of each: the muscles and ligaments

concerned serving here, as in other cases, so to hold the bones that

they bear the pressure instead of being displaced by it. Not forgetting

that many processes of the bones have to bear tensions, we may then

say that generally, though by no means universally, bones are internal

dense masses that have to bear pressures--pressures which in the

cylindrical bones become longitudinal thrusts. Leaving out exceptional

cases, let us consider bones as masses thus circumstanced.

When giving reasons for the belief that the vertebrate skeleton is

mechanically originated, one of the facts put in evidence was, that

in the vertebrate series the transition from the cartilaginous to the

osseous spine begins peripherally (§ 257): each vertebra being at first

a ring of bone surrounding a mass of cartilage. And it was pointed

out that this peripheral ossification is ossification at the region

of greatest pressures. Now it is not vertebræ only that follow this

course of development. In a cylindrical bone, though it is differently

circumstanced, the places of commencing ossification are still the

places on which the severest stress falls. Let us consider how such a

bone that has to bear a longitudinal pressure is mechanically affected.

If the end of a walking-cane be thrust with force against the ground,

the cane bends; and partially resuming its straightness when relieved,

again bends, usually towards the same side, when the thrust is renewed.

A bend so caused acts on the fibres of the cane in nearly the same

way as does a bend caused by supporting the cane horizontally at its

two ends and suspending a weight from its middle. In either case the

fibres on the convex side are extended and the fibres on the concave

side compressed. Kindred actions occur in a rod that is so thick

as not to yield visibly under the force applied. In the absence of

complete homogeneity of its substance, complete symmetry in its form,

and an application of a force exactly along its axis, there must be

some lateral deflection; and therefore some distribution of tensions

and pressures of the kind indicated. And then, as the fact which here

specially concerns us, we have to note that the strongest tensions

and pressures are borne by the outer layers of fibres. Now the shaft

of a long bone, subject to mechanical actions of this kind, similarly

has its outer layer most strained. In this layer, therefore, on the

mechanical hypothesis, ossification should commence, and here it does

commence--commences, too, midway between the ends, where the bends

produce on the superficial parts their most intense effects. But we

have not in this place simply to observe that ossification commences at

the places of greatest stress, but to ask what causes it to do this.

Can we trace the physical actions which set up this deposit of dense

tissue? It is, I think, possible to indicate a “true cause” that is at

work; though whether it is a sufficient cause may be questioned. We

concluded that in certain other cases, the formation of dense tissue

indirectly results from the alternate squeezing and relaxation of the

vessels running through the part; and the inquiry now to be made is,

whether, in developing bone, the same actions go on in such ways as to

produce the observed effects. At the outset we are met by what seems a

fatal difficulty--cartilage is a non-vascular tissue: this substance

of which unossified bones consist is not permeated by minute canals

carrying nutritive liquid, and cannot, therefore, be a seat of actions

such as those assigned. This apparent difficulty, however, furnishes a

confirmation. For cartilage that is wholly without permeating canals

does not ossify: ossification takes place only at those parts of it

into which the canals penetrate. Hence, we get additional reason for

suspecting that bone-formation is due to the alleged cause; since it

occurs where mechanical strains can produce the actions described, but

does not occur where mechanical strains cannot produce them. Let us

consider more closely what the several factors are. It will suffice

for the argument if we commence with the external vascular layer as

already existing, and consider what will take place in it. Cartilage

is elastic--is somewhat extensible, and spreads out laterally under

pressure, but resumes its form when relieved. How, then, will the

minute channels traversing it in all directions be affected at the

places where it is strained by a bend? Those on the convex side will

be laterally squeezed, in the same way that we saw the sap-vessels on

the convex side of a bent branch are squeezed; and as exudation of the

sap into the adjacent prosenchyma will be caused in the one case, so,

in the other, there will be caused exudation of serum into the adjacent

cartilage: extra nutrition and increase of strength resulting in both

cases. The parallel ceases here, however. In the shoot of a plant, bent

in various directions by the wind, the side which was lately compressed

is now extended; and hence that squeezing of the sap-vessels which

results from extension, suffices to feed and harden the tissue on all

sides of the shoot. But it is not so with a bone. Having yielded on

one side under longitudinal pressure, and resumed as nearly as may be

its previous shape when the pressure is taken off, the bone yields

again towards the same side when again longitudinally pressed. Hence

the substance of its concave side, never rendered convex by a bend

in the opposite direction, would not receive any extra nutrition did

no other action come into play. But if we consider how intermittent

pressures must act on cartilage, we shall see that there will result

extra nutrition of the concave side also. Squeeze between two pieces of

glass a thin bit of caoutchouc which has a hole through it. While the

caoutchouc spreads out away from the centre, it also spreads inwards,

so as partially to close the hole. Everywhere its molecules move away

in directions of least resistance; and for those near the hole, the

direction of least resistance is towards the hole. Let this hole stand

for the transverse section of one of the minute canals or channels

passing through cartilage, and it will be manifest that on the side of

the unossified bone made concave in the way described, the compressed

cartilage will squeeze the canals traversing it; and, in the absence

of perfect homogeneity in the cartilage, the squeeze will cause extra

exudation from the canals into the cartilage. Thus every additional

strain will give to the cartilage it falls upon, an additional supply

of the materials for growth. So that presently the side which, by

yielding more than any other, proves itself to be the weakest, will

cease to be the weakest. What further will happen? Some other side

will yield a little--the bends will take place in some other plane;

and the portions of cartilage on which repeated tensions and pressures

now fall will be strengthened. Thus the rate of nutrition, greatest at

the place where the bending is greatest, and changing as the incidence

of forces changes, will bring about at every point a balance between

the resistances and the strains. Thus, too, there will be determined

that peripheral induration which we see in bones so circumstanced.

As in a shoot we saw that the woody deposit takes place towards the

outside of the cylinder, where, according to the hypothesis, it ought

to take place; so, here, we see that the excess of exudation and

hardening, occurring where the strains are most intense, will form a

cylinder having a dense outside and a porous or hollow inside. These

processes will be essentially the same in bones subject to more complex

mechanical actions, such as sundry of the flat bones and others that

serve as internal fulcra. Be the strains transverse or longitudinal,

be they torsion strains or mixed strains, the outer parts of the

bone will be more affected by them than its inner parts. They will

therefore tend everywhere to produce resisting masses having outer

parts more dense than their inner parts. And by causing most growth

where they are most intense, they will call out reactive forces

adequate to balance them. There are doubtless obstacles in the way

of this interpretation. It may be said that the forces acting on the

outer layers in the manner described, would compress the canals too

little to produce the alleged effects; and if evenly distributed along

the whole lengths of the layers, they would probably do so. But it

needs only to bend a flexible mass and observe the tendency to form

creases on the concave surface, to feel assured that along the surface

of an ossifying bone, the yielding of the tissue when bent will not

be uniform. In the absence of complete homogeneity, the interstitial

yielding will take place at some points more than others, and at one

point above all others. When, at the weakest point--the centre of

commencing ossification--an extra amount of deposit has been caused,

it will cease to be the weakest; and adjacent points, now the weakest,

will become the places of yielding and induration. It may be further

objected that the hypothesis is incompatible with the persistence

of cartilage for so long a time between the epiphysis of bones and

the bony masses which they terminate. But there is the reply that

the places occupied by this cartilage being places at which the bone

lengthens, the non-ossification is in part apparent only--it is rather

that new cartilage is formed as fast as the pre-existing cartilage

ossifies; and there is the further reply that the slowness of the

ultimate ossification of this part, is due to its non-vascularity, and

to mechanical conditions which are unfavourable to its acquirement of

vascularity. Once more, there is the demurrer that in the epiphyses

ossification does not begin at the surface but within the mass of

the cartilage. Explanation of this implies ability to follow out the

mechanical actions in a resilient substance which, like india-rubber,

admits of being distorted in all ways by pressure and recovering its

form, and it seems impossible to say how the more superficial and more

deep-seated canals traversing it will be respectively affected.

Of course it is not meant that this osseous development by direct

equilibration takes place in the individual. Though it is a corollary

from the argument that in each individual the process must be furthered

and modified by the particular actions to which the particular bones

are exposed; yet the leading traits of structure assumed by the bones

are assumed in conformity with the inherited type. This, however, is

no difficulty. The type itself is to be regarded as the accumulated

result of such modifications, transmitted and increased from generation

to generation. The actions above described as taking place in the

bone of an individual, must be understood as producing their total

effect little by little in the corresponding bones of a long series of

individuals. Even if but a small modification can be so wrought in the

individual, yet if such modification, or a part of it, is inheritable,

we may readily understand how, in the course of geologic epochs, the

observed structures may arise in the assigned way.

Here may fitly be added a strong confirmation. If we find cases

where individual bones, subject in exceptional degrees to the

actions described, present in exceptional amounts the modifications

attributed to them, we are greatly helped in understanding how there

may be produced in the race that aggregate of modifications which

the hypothesis implies. Such cases occur in ricketty children. I

am indebted to Mr. Busk for pointing out these abnormal formations

of dense tissue, that are not apparently explicable as results of

mechanical actions and reactions. It was only on tracing out the

processes here at work, that there suggested itself the specific

interpretation of the normal process, as above set forth. When, from

constitutional defect, bones do not ossify with due rapidity, and are

meanwhile subject to the ordinary strains, they become distorted.

Remembering how a mass which has been made to yield in any direction

by a force it cannot withstand, is some little time before it recovers

completely its previous form, and usually, indeed, undergoes what

is called a “permanent set;” it is inferable that when a bone is

repeatedly bent at the same time that the liquid contained in its

canals is poor in the materials for forming dense tissue, there

will not take place a proportionate strengthening of the parts most

strained; and these parts will give way. This happens in rickets. But

this having happened, there goes on what, in teleological language,

we call a remedial process. Supposing the bone to be one commonly

affected--a femur; and supposing a permanent bend to have been caused

in it by the weight of the body; the subsequent result is an unusual

deposition of cartilaginous and osseous matter on the concave side of

the bone. If the bone is represented by a strung bow, then the deposit

occurs at the part represented by the space between the bow and the

string. And thus occurring where its resistance is most effective,

it increases until the approximately-straight piece of bone formed

within the arc, has become strong enough to bear the pressure without

appreciably yielding. Now this direct adaptation, seeming so like a

special provision, and furnishing so remarkable an instance of what,

in medical but unscientific language, is called the _vis medicatrix

naturæ_, is simply a result of the above-described mechanical actions

and reactions, going on under the exceptional conditions. Each time

such a bent bone is subject to a force which again bends it, the

severest compression falls on the substance of its concave side. Each

time, then, the canals running through this part of its substance are

violently squeezed--far more squeezed than they or any other of the

canals would have been, had the bone remained straight. Hence, on every

repetition of the strain, these canals near the concave surface have

their contents forced out in more than normal abundance. The materials

for the formation of tissue are supplied in quantity greater than can

be assimilated by the tissue already formed; and from the excess of

exuded plasma, new tissue arises.[50] A layer of organizable material

accumulates between the concave surface and the periosteum; in this,

according to the ordinary course of tissue-growth, new vessels appear;

and the added layer presently assumes the histological character of the

layer from which it has grown. What next happens? This added layer,

further from the neutral axis than that which has thrown it out, is now

the most severely compressed, and its vessels are the most severely

squeezed. The place of greatest exudation and most rapid deposit

of matter, is therefore transferred to this new layer; and at the

same time that active nutrition increases its density, the excess of

organizable material forms another layer external to it: the successive

layers so added, encroaching on the space between the concave surface

of the bone and the chord of its arc. What limits the encroachment on

this space?--what stops the process of filling it up? The answer to

this question will be manifest when observing that there comes into

play a cause which gradually diminishes the forces falling on each new

layer. For the transverse sectional area is step by step increased; and

an increase of the area over which the weight borne is distributed,

implies a relatively smaller pressure upon each part of it. Further, as

the transverse dimensions of the bone increase, the materials composing

its convex and concave layers, becoming further from the neutral axis,

become better placed for resisting the strains to be borne. So that

both by the increased quantity of dense matter and by its mechanically

more-advantageous position, the bendings of the bone are progressively

decreased. But as they are decreased, each new layer formed on the

concave surface has its substance and its vessels less compressed; and

the resulting growth and induration are rendered less rapid. Evidently,

then, the additions, slowly diminishing, will eventually cease; and

this will happen when the bone no longer bends. That is to say, the

thickening of the bone will reach its limit when there is equilibrium

between the incident forces and the forces which resist them. Here,

indeed, we may trace with great clearness the process of direct

equilibration--may see how an unusual force, falling on the moving

equilibrium of an organism and not overthrowing it, goes on working

modifications until the reaction balances the action.

That, however, which now chiefly concerns us, is to note how this

marked adaptation supports the general argument. Unquestionably bone

is in this case formed under the influence of mechanical stress, and

formed just where it most effectually meets the stress. This result,

not otherwise explained, is explained by the hypothesis above set

forth. And when we see that this special deposit of bone is accounted

for by actions like those to which bone-formation in general is

ascribed, the probability that these are the actions at work becomes

very great.[51]

Of course it is not alleged that osseous structures arise in this

way alone. The bones of the skull and various dermal bones cannot be

thus interpreted. Here the natural selection of favourable variations

appears the only assignable cause--the equilibration is indirect. We

know that ossific deposits now and then occur in tissues where they

are not usually found; and such deposits, originally abnormal, if they

occurred in places where advantages arose from them, might readily be

established and increased by survival of the fittest. Especially might

we expect this to happen when a constitutional tendency to form bone

had been established by actions of the kind described; for it is a

familiar fact that differentiated types of tissue, having once become

elements of an organism, are apt occasionally to arise in unusual

places, and there to repeat all their peculiar histological characters.

And this may possibly be the reason why the bones of the skull, though

not exposed to forces such as those which produce, in other bones,

dense outer layers including less dense interiors, nevertheless repeat

this general trait of bony structure. While, however, it is beyond

doubt that some bones are not due to the direct influence of mechanical

stress, we may, I think, conclude that mechanical stress initiates

bone-formation.

§ 302. What is the origin of nerve? In what way do its properties stand

related to the properties of that protoplasm whence the tissues in

general arise? and in what way is it differentiated from protoplasm

simultaneously with the other tissues? These are profoundly interesting

questions; but questions to which positive answers cannot be expected.

All that can be done is to indicate answers which seem feasible.

That the property specially displayed by nerve, is a property which

protoplasm possesses in a lower degree, is manifest. The sarcode

of a Rhizopod and the substance of an unimpregnated ovum, exhibit

movements that imply a propagation of stimulus from one part of

the mass to another. We have not far to seek for a probable origin

of this phenomenon. There is good reason for ascribing it to the

extreme instability of the organic colloids of which protoplasm

consists. These, in common with colloids in general, assume

different isomeric forms with great facility; and they display not

simply isomerism but polymerism. Further, this readiness to undergo

molecular re-arrangement, habitually shows itself in colloids by

the rapid propagation of the re-arrangement from part to part. As

Prof. Graham has shown, matter in this state often “pectizes” almost

instantaneously--a touch will transform an entire mass. That is to say,

the change of molecular state once set up at one end, spreads to the

other end--there is a progress of a stimulus to change; and this is

what we see in a nerve. So much being understood, let us re-state the

case more completely.

Molecular change, implying as it does motion of molecules, communicates

motion to adjacent molecules; be they of the same kind or of a

different kind. If the adjacent molecules, either of the same kind or

of a different kind, be stable in composition, a temporary increase

of oscillation in them as wholes, or in their parts, may be the only

result; but if they are unstable there are apt to arise changes of

arrangement among them, or among their parts, of more or less permanent

kinds. Especially is this so with the complex molecules which form

colloidal matter, and with the organic colloids above all. Hence it is

to be inferred that a molecular disturbance in any part of a living

animal, set up by either an external or internal agency, will almost

certainly disturb and change some of the surrounding colloids not

originally implicated--will diffuse a wave of change towards other

parts of the organism: a wave which will, in the absence of perfect

homogeneity, travel further in some directions than in others. Let us

ask next what will determine the differences of distance travelled in

different directions. Obviously any molecular agitation spreading from

a centre, will go furthest along routes that offer least resistance.

What routes will these be? Those along which there lie most molecules

that are easily changed by the diffused molecular motion, and which

yet do not take up much molecular motion in assuming their new states.

Molecules which are tolerably stable will not readily propagate the

agitation; for they will absorb it in the increase of their own

oscillations, instead of passing it on. Molecules which are unstable

but which, in assuming isomeric forms, absorb motion, will not readily

propagate it; since it will disappear in working the changes in them.

But unstable molecules which, in being isomerically transformed, do not

absorb motion, and still more those which, in being so transformed,

give out motion, will readily propagate any molecular agitation; since

they will pass on the impulse either undiminished, or increased, to

adjacent molecules. If then we assume, as we are not only warranted

in doing but are obliged to do, that protoplasm contains two or more

colloids, either mingled or feebly combined (since it cannot consist of

simple albumen or fibrin or casein, or any allied proximate principle);

it may be concluded that any molecular agitation set up by what we call

a stimulus, will diffuse itself further along some lines than along

others, if the components of the protoplasm are not quite homogeneously

dispersed, and if some of them are isomerically transformed more

easily, or with less expenditure of motion, than others; and it will

especially travel along spaces occupied chiefly by those molecules

which give out molecular motion during their metamorphoses, if there

should be any such. But now let us ask what structural effects

will be wrought along a tract traversed by this wave of molecular

disturbance. As is shown by those transformations which so rapidly

propagate themselves through colloids, molecules that have undergone a

certain change of form, are apt to communicate a like change of form

to adjacent molecules of the same kind--the impact of each overthrow

is passed on and produces another overthrow. Probably the proneness

towards isochronism of molecular movements necessitates this. If any

molecule has had its components re-arranged, and their oscillations

consequently altered, there result movements not concordant with the

movements in adjacent untransformed molecules, but which, impressing

themselves on the parts of such untransformed molecules, tend to

generate in them concordant movements--tend, that is, to produce

the re-arrangements involved by these concordant movements. Is this

action limited to strictly isomeric substances? or may it extend to

substances that are closely-allied? If along with the molecules of

a compound colloid there are mingled those of some kindred colloid;

or if with the molecules of this compound colloid there are mingled

the components out of which other such molecules may be formed; then

there arises the question--does the same influence which tends to

propagate the isomeric transformations, tend also to form new molecules

of the same kind out of the adjacent components? There is reason to

suspect that it does. Already when treating of the nutrition of parts

(§ 64), it was pointed out that we are obliged to recognize a power

possessed by each tissue to build up, out of the materials brought

to it, molecules of the same type as those of which it is formed.

This building up of like molecules seems explicable as caused by the

tendency of the new components which the blood supplies, to acquire

movements isochronous with those of the like components in the tissue;

which they can do only by uniting into like compound molecules.

Necessarily they must gravitate towards a state of equilibrium; such

state of equilibrium--moving equilibrium of course--must be one in

which they oscillate in the same times with neighbouring molecules;

and so to oscillate they must fall into groups identical with the

groups around them. If this be a general principle of tissue-growth

and repair, we may conclude that it will apply in the case before

us. A wave of molecular disturbance passing along a tract of mingled

colloids closely-allied in composition, and isomerically transforming

the molecules of one of them, will be apt at the same time to form

some new molecules of the same type, at any place where there exist

the proximate components, either uncombined or feebly combined in some

not very different way. And this will be most likely to occur where

the molecules of the colloid that are undergoing the isomeric change,

predominate, but have scattered through them the other molecules out of

which they may be formed, either by composition or modification. That

is to say, a wave of molecular disturbance diffused from a centre, and

travelling furthest along a line where lie most molecules that can be

isomerically transformed with facility, will be likely at the same time

to further differentiate this line, and make it more characterized than

before by the easy-transformability of its molecules. One additional

step, and the interpretation is reached. Analogy shows it to be not

improbable that these organic colloids, isomerically transformed by

slight molecular impact or increase of molecular motion, will some of

them resume their previous molecular structures after the disturbance

has passed. We know that what are stable molecular arrangements under

one degree of molecular agitation, are not stable under another degree;

and there is evidence that re-arrangements of an inconspicuous kind

are occasionally brought about by very slight changes of molecular

agitation. Water supplies a clear case. Prof. Graham infers that water

undergoes a molecular re-arrangement at about 32°--that ice has a

colloid form as well as a crystalloid form, dependent on temperature.

Send through it an extra wave of the molecular agitation we call heat,

and its molecules aggregate in one way. Let the wave die away, and its

molecules resume their previous mode of aggregation. And obviously

such transformations may be repeated backwards and forwards within

narrow limits of temperature. Now among the extremely unstable organic

colloids, such a phenomenon is far more likely to happen. Suppose,

then, that the nerve-colloid is one of which the molecules are changed

in form by a passing wave of extra agitation, but resume their previous

form when the wave has passed: the previous form being the most stable

under the conditions which then recur. What follows? It follows that

these molecules will be ready again to undergo isomeric transformation

when there again occurs the stimulus; will, as before, propagate the

transformation most along the tract where such molecules are most

abundant; will, as before, tend to form new molecules of their own

type; will, as before, make the line along which they lie one of easier

transfer for the molecular agitation. Every repetition will help to

increase, to integrate, to define more completely, the course of the

escaping molecular motion--extending its remoter part while it makes

its nearer part more permeable--will help, that is, to form a line of

discharge, a line for conducting impressions, a nerve.

Such seems to me a not unfair series of deductions from the known

habitudes of colloids in general and the organic colloids in

particular. And I think that the implied nature and properties of nerve

correspond better with the observed phenomena than do the nature and

properties implied by other hypotheses. Of course the speculation as

it here stands is but tentative, and leaves much unexplained. It gives

no obvious reply to the questions--what causes the formation of nerves

in directions adapted to the needs? what determines their appropriate

connexions?--questions, however, to which, when we come to deal with

physiological integration, we may find not unsatisfactory answers.

Moreover it says nothing about the genesis of ganglia. A ganglion,

it is clear, must consist of a colloidal matter equally unstable, or

still more unstable, which, when disturbed, falls into some different

molecular arrangement, perhaps chemically simpler, and gives out in

so doing a large amount of molecular motion--serves as a reservoir of

molecular motion which may be suddenly discharged along an efferent

nerve or nerves, when excitement of an afferent nerve has disengaged

it. How such a structure as this results, the hypothesis does not show.

But admitting these shortcomings it may still be held that we are, in

the way pointed out, enabled to form some idea of the actions by which

nervous tissue is differentiated.

§ 303. A speculation akin to, and continuous with, the last,

is suggested by an inquiry into the origin of muscular tissue.

Contractility as well as irritability is a property of protoplasm

or sarcode; and, as before suggested (§ 22), is not improbably

due to isomeric change in one or more of its component colloids.

It is a feasible supposition that of the several isomeric changes

simultaneously set up among these component colloids, some may be

accompanied by change of bulk and some not. Clearly the isomeric

change undergone by the colloid which we suppose to form nerve, must

be one not accompanied by appreciable change of bulk; since change of

bulk implies “internal work,” as physicists term it, and therefore

expenditure of force. Conversely, the colloid out of which muscle

originates, may be one that readily passes into an isomeric state

in which it occupies less space: the molecular disturbance causing

this contraction being communicated to it from adjacent portions of

nerve-substance that are molecularly disturbed; or being otherwise

communicated to it by direct mechanical or chemical stimuli: as happens

where nerves do not exist, or where their influence has been cut off.

This interpretation seems, indeed, to be directly at variance with

the fact that muscle does not diminish in bulk during contraction

but merely changes its shape. That which we see take place with the

muscle as a whole, is said also to take place with each fibre--while

it shortens it also broadens. There is, however, a possible solution

of this difficulty. A contracting colloid yields up its water; and

the contracted colloid _plus_ the free water, may have the same bulk

as before though the colloid has less. If it be replied that in this

case the water should become visible between the substance of the

fibre and its sarcolemma or sheath, it may be rejoined that this

is not necessary--it may be deposited interstitially. Possibly the

striated structure is one that facilitates its exudation and subsequent

re-absorption; and to this may be due the superiority of striated

muscle in rapidity of contraction. Granting the speculative character

of this interpretation, let us see how far it agrees with the facts.

If the actions are as here supposed, the contracted or more integrated

state of the muscular colloid will be that which it tends continually

to assume--that into which it has an increasing aptitude to pass when

artificial paralysis has been produced, as shown by Dr. Norris--that

into which it lapses completely in _rigor mortis_. The sensible motion

generated by the contraction can arise only from the transformation

of insensible motion. This insensible motion suddenly yielded up by

a contracting mass, implies the fall of its component molecules into

more stable arrangements. And there can be no such fall unless the

previous arrangement is unstable. From this point of view, too, it is

possible to see how the hydro-carbons and carbo-hydrates consumed in

muscular action, may produce their effects. For these non-nitrogenous

elements of food, when consumed in the tissues, give out large amounts

of molecular motion. They do this in presence of the muscular colloids

which have lost molecular motion during their fall in the stable or

contracted state. From the molecular motion they give out, may be

restored the molecular motion lost by the contracted colloids; and

these contracted colloids may thus have their molecules raised to that

unstable state from which, again falling, they can again generate

mechanical motion.

This conception of the nature and mode of action of muscle, while it

is suggested by known properties of colloidal matter and conforms to

the recent conclusions of organic chemistry and molecular physics,

establishes a comprehensible relation between the vital actions of

the lower and the higher animals. If we contemplate the movements

of cilia, of a Rhizopod’s pseudopodia, of a Polype’s body, or of the

long pendant tentacles of a _Medusa_, we shall see great congruity

between them and this hypothesis. Bearing in mind that the contractile

substance of developed muscle is affected not by nervous influence

only, but, where nervous influence is destroyed, is made to contract

by mechanical disturbance and chemical action, we may infer that

it does not differ intrinsically from the primordial contractile

substance which, in the lowest animals, changes its bulk under other

stimuli than the nervous. We shall see significance in the fact

ascertained by Dr. Ransom, that various agents which excite and arrest

nervo-muscular movements in developed animals, excite and arrest the

protoplasmic movements in ova. We shall understand how tissues not yet

differentiated into muscle and nerve, have this joint irritability and

contractility; how muscle and nerve may arise by the segregation of

their mingled colloids, the one of which, not appreciably altering its

bulk during isomeric change, readily propagates molecular disturbance,

while the other, contracting when isomerically changed, less readily

passes on the molecular disturbance; and how, by this differentiation

and integration of the conducting and the contracting colloids, the one

ramifying through the other, it becomes possible for a whole mass to

contract suddenly, instead of contracting gradually, as it does when

undifferentiated.