CHAPTER V.EVOLUTION.
99.Thatorganized substance has the property of nourishing itself by assimilating from its internal medium substances there present in an unorganized state, and that this is followed by a development or differentiation of structure, is familiar to every inquirer.
Every one who has pursued embryological researches, and in a lesser degree every one who has merely read about them, must have been impressed by this marvel of marvels: an exceedingly minute portion of living matter, so simple in aspect that a line will define it, passes by successive modifications into an organism so complex that a treatise is needed to describe it; not only do the cells in which the ovum and the spermatozoon originate, pass into a complex organism, reproducing the forms and features of the parents, and with these the constitutional peculiarities of the parents (their longevity, their diseases, their mental dispositions, nay, their very tricks and habits), but they may reproduce the form and features, the dispositions and diseases, of a grandfather or great-grandfather, which had lain dormant in the father or mother. Consider for an instant what this implies. A microscopic cell of albuminous compounds, wholly without trace of organs, not appreciably distinguishable from millions of other cells, does nevertheless contain within it the “possibilities” of an organism so complex and so special as that of a Newton or a Napoleon. If ever there was a casewhen the famous Aristotelian notion of a “potential existence” seemed justified, assuredly it is this. And although we can only by a fallacy maintain the oak to becontainedin the acorn, or the animal contained in the ovum, the fallacy is so natural, and indeed so difficult of escape, that there is no ground for surprise when physiologists, on first learning something of development, were found maintaining that the perfect organism existed already in the ovum, having all its lineaments in miniature, and only growing into visible dimensions through the successive stages of evolution.44The preformation of the organism seemed an inevitable deduction from the opinions once universal. It led to many strange, and some absurd conclusions; among them, to the assertion that the original germ of every species contained within it all the countless individuals which in process of time might issue from it; and this in no metaphysical “potential” guise, but as actual boxed-up existences (emboîtés); so that Adam and Eve were in the most literal sense progenitors of the whole human race, and contained their progeny already shaped within them, awaiting the great accoucheur, time.
100. This was the celebrated “emboîtement” theory. In spite of obvious objections it gained scientific acceptance, because physiologists could not bring themselves to believe that so marvellous a structure as that of a human organism arose by a series of successive modifications, or because they could not comprehend how it was built up, part by part, into forms so closely resembling the parent-forms. That many and plausible reasons pleaded in favor of this opinion is evident in the fact that illustrious men like Haller, Bonnet, Vallisneri, Swammerdamm, Réaumur, and Cuvier, were its advocates; and if there is not a siglephysiologist of our day who accepts it, or who finds any peculiar difficulty in following the demonstrations of embryologists, how from the common starting-point of a self-multiplying epithelial cell parts so diverse as hairs, nails, hoofs, scales, feathers, crystalline lens, and secreting glands may be evolved, or how from the homogeneous germinal membrane the complex organism will arise, there are very few among the scorners of the dead hypothesis who seem capable of generalizing the principles which have destroyed it, or can conceive that the laws of Evolution apply as rigorously to the animal and vegetablekingdomsas to the individualorganisms. The illustrious names of those who advocated the preformation hypothesis may serve to check our servile submission to the authorities so loudly proclaimed as advocates of the fixity of species. The more because the two doctrines have a common parentage. The one falls with the other, and no array of authorities can arrest the fall. That the manifold differentiations noticeable in a complex organism should have been evolved from a membrane wholly destitute of differences is a marvel, but a marvel which Science has made intelligible. Yet the majority of those to whom this has been made intelligible still find an impossibility in admitting that the manifold forms of plant and animal were successively evolved from equally simple origins. They relinquish the hypothesis of preformation in the one case, and cling to it in the other. Evolution, demonstrable in the individual history, seems preposterous in the history of the class. And thus is presented the instructive spectacle of philosophers laughing at the absurdities of “preformation,” and yet exerting all their logic and rhetoric in defence of “creative fiats”—which is simply the preformation hypothesis “writ large.”
101. It would not be difficult to show that the doctrineof Epigenesis, with which Wolff forever displaced the doctrine of Preformation, leads by an inevitable logic to the doctrine of universal Evolution; and that we can no more understand the appearance of a new organism which is not the modification of some already existing organism, than we can understand the sudden appearance of a new organ which is not the modification of some existing structure. In the one case as in the other we may disguise the process under such terms as creative fiat and preformation; but these terms are no explanations; they re-state the results, they do not describe the process; whereas Epigenesis describes the process as it passes under the eye of science.
102. If any reader of these pages who, from theological or zoölogical suspicion of the Development Hypothesis, clings to the hypothesis of a creative Plan which once for all arranged the organic world in Types that could not change, will ask what rational interpretation can be given to the succession of phases each embryo is forced to pass through, it may help to give him pause. He will observe thatnoneof these phases have any adaptation to the future state of the animal, but are in positive contradiction to it, or are simply purposeless; whereas all show stamped on them the unmistakable characters ofancestraladaptations and the progressions of Organic Evolution. What does the fact imply? There is not a single known example of a complex organism which is not developed out of simpler forms. Before it can attain the complex structure which distinguishes it, there must be an evolution of forms similar to those which distinguish the structures of organisms lower in the series. On the hypothesis of a Plan which prearranged the organic world, nothing could be more unworthy of a supreme intelligence than this inability to construct an organism at once, without previously making severaltentative efforts, undoing to-day what was so carefully done yesterday, andrepeating for centuries the same tentatives, and the same corrections, in the same succession. Do not let us blink this consideration. There is a traditional phrase much in vogue among the anthropomorphists, which arose naturally enough from the tendency to take human methods as an explanation of the divine—a phrase which becomes a sort of argument—“The Great Architect.” But if we are to admit the human point of view, a glance at the facts of embryology must produce very uncomfortable reflections. For what should we say to an architect who was unable, or being able was obstinately unwilling, to erect a palace except by first using his materials in the shape of a hut, then pulling it down and rebuilding them as a cottage, then adding story to story and room to room,notwith any reference to the ultimate purposes of the palace, but wholly with reference to the way in which houses were constructed in ancient times? What should we say to the architect who could not form a museum out of bricks and mortar, but was forced to begin as if going to build a mansion: and after proceeding some way in this direction, altered his plan into a palace, and that again into a museum? Yet this is the sort of succession on which organisms are constructed. The fact has long been familiar; how has it been reconciled with Infinite Wisdom? Let the following passage answer for a thousand:—“The embryo is nothing like the miniature of the adult. For a long while the body in its entirety and its details presents the strangest of spectacles. Day by day and hour by hour the aspect of the scene changes, and this instability is exhibited by the most essential parts no less than by the accessory parts. One would say that Nature feels her way, and only reaches the goal after many times missing the path,—on dirait que la nature tâtonne et ne conduitson œuvre à bon fin qu’après s’être souvent trompée.”45Writers have no compunction in speaking of Nature feeling her way and blundering; but if in lieu of Nature, which may mean anything, the Great Architect be substituted, it is probable that the repugnance to using such language of evasion may cause men to revise their conceptions altogether; they dare not attribute ignorance and incompetence to the Creator.
103. Obviously the architectural hypothesis is incompetent to explain the phenomena of organic development. Evolution is the universal process; not creation of a direct kind. Von Baer, who very properly corrected the exaggerations which had been put forth respecting the identity of the embryonic forms with adult forms lower in the scale, who showed that the mammalian embryo never was a bird, a reptile, or a fish, nevertheless emphasized the fact that the mammalian embryo passes through all the lower typical forms; so much so that, except by their size, it is impossible to distinguish the embryos of mammal, bird, lizard, or snake. “In my collection,” he says, “there are two little embryos which I have omitted to label, so that I am now quite incompetent to say to what class they belong. They may be lizards, they may be small birds, or very young mammals; so complete is the similarity in the mode of formation of the head and trunk. The extremities have not yet made their appearance. But even if they existed in the earliest stage we should learn nothing from them, for the feet of lizards, mammals, and the wings of birds, all arise from the same common form.” He sums up with his formula: “The special type is always evolved from a more general type.”46
Such reminiscences of earlier forms are intelligible on the supposition that originally the later form was a modification of the earlier form, and that this modification is repeated; or on the supposition that there was a similarity in the organic conditions, which similarity ceased at the point where the new form emerged. But on no hypothesis of creative Plan are they intelligible. They are useless structures, failing even to subserve a temporary purpose. Sometimes, as Mr. Darwin remarks, a trace of the embryonic resemblance lasts till a late age: “Thus birds of the same genus, and of closely allied genera, often resemble each other in their first and second plumage: as we see in the spotted feathers in the thrush group. In the cat tribe most of the species are striped and spotted in lines; and stripes or spots can plainly be distinguished in the whelp of the lion and the puma. We occasionally, though rarely, see something of this kind in plants.... The points of structure in which the embryos of widely different animals of the same class resemble each other often have no direct relation to their conditions of existence. We cannot, for instance, suppose that in the embryos of the vertebrata the peculiar loop-like courses of the arteries near the bronchial slits are related to similar conditions in the young mammal which is nourished in the womb of its mother, in the egg of a bird which is hatched in a nest, and in the spawn of a frog under water.”
104. It would be easy to multiply examples, but I will content myself with three. The tadpole of the Salamander has gills, and passes his existence in the water; but theSalamandra atra, which lives high up among the mountains, brings forth its young full-formed. This animal never lives in the water. Yet if we open a gravid female, we find tadpoles inside her with exquisitely feathered gills, and (as I have witnessed) these tadpoles “whenfrom the mother’s womb untimely ripped,” if placed in water, swim about like the tadpoles of water newts. Obviously this aquatic organization has no reference to the future life of the animal, nor has it any adaptation to its embryonic condition; it has solely reference to ancestral forms, it repeats a phase in the development of its progenitors. Again, in the embryo of the naked Nudibranch, we always observe a shell, although the animal is without a shell, and there can be no purpose served by the shell in embryonic life.47Finally, the human embryo has a tail, which is of course utterly purposeless, and which, although to be explained as a result of organic laws, is on the creative hypothesis only explained as an adherence to the general plan of structure—a specimen of pedantic trifling “worthy of no intellect above the pongo’s.”48
105. Humanly appreciated, not only is it difficult to justify the successive stages of development, the incessant building up of structures immediately to be taken down, but also to explain why development was necessaryat all. Why are not plants and animals formed at once, as Eve was mythically affirmed to be taken from Adam’s rib, and Minerva from Jupiter’s head? The theory of Evolution answers this question very simply; the theory of Creation can only answer it by affirming that such was the ordained plan. But the theory of Evolution not only gives the simpler and more intelligible answer to this question, it gives an answer to the further question which leaves the theory of Creation no loophole except a sophism—namely, why the formation of organisms is constantly being frustrated or perverted? And, further, it gives an explanation of the law noticed by Milne Edwards, that Nature is as economical in her means as she is prodigal in her variation of them: “On dirait qu’avant de recourir à des ressources nouvelles elle a voulu épuiser, en quelque sorte, chacun des procédés qu’elle avait mis en jeu.”49The applause bestowed on Nature for being economical is a curious transference to Nature of human necessities. Why, with a whole universe at her disposal, should Nature be economical? Why must she always be working in the same groove, and using but a few out of the many substances at her command? Economy is a virtue only in the poor. If Nature, in organic evolutions, is restricted to a very few substances, and a very few modes of combination, always creating new forms by modification of the old, and apparently incapable of creating an organism at once, this must imply an inherent necessity which is very unlike the free choice that can render economy a merit.
106. There may indeed be raised an objection to the Development Hypothesis on the ground that if the complex forms were all developed from the simpler forms, we ought to trace the identities through all their stages. If the fish developed into the reptile, the reptile into thebird, and the bird into the mammal (which I, for one, think questionable), we ought to find, it is urged, evidence of this passage. And at one time it was asserted that the evidence existed; but this has been disproved, and on the disproof the opponents of Evolution take their stand. Although I cannot feel much confidence in the idea of such a passage from Type to Type, and although the passage, if ever it occurred, must have occurred at so remote a period as to leave no evidence more positive than inference, I cannot but think the teaching of Embryology far more favorable to it than to our opponents. Supposing, for the sake of argument, that the passage did take place, ought we to find the embryonic stages accurately reproducing the permanent forms of lower types? Von Baer thinks we ought; and lesser men may follow him without reproach. But it seems to me that he starts from an inadmissible assumption, namely, that the development must necessarily be in a straight line rather than in a multiplicity of divergent lines. “When we find the embryonic condition,” he says, “differing from the adult, we ought to find a corresponding condition somewhere in the lower animals.”50Not necessarily. We know that the mental development of a civilized man passes through the stages which the race passed through in the course of its long history, and the psychology of the child reproduces the psychology of the savage. But as this development takes place under conditions in many respects different, and as certain phases are hurried over, we do not expect to find a complete parallel. It is enough if we can trace general resemblances. Von Baer adds, “That certain correspondences should occur between the embryonic states of some animals and the adult states of others seems inevitable and of no significance(?). They could not fail, since the embryos lie within the animal sphere,and the variations of which the animal body is capable are determined for each type by the internal connection and mutual reaction of its organs, so that particular repetitions are inevitable.” A profound remark, to which I shall hereafter have occasion to return, but its bearing on the present question is inconclusive. The fact that the embryonic stages of the higher animals resemble in general characters the permanent stages of the lower animals, and very closely resemble the embryonic stages of those animals, is all that the Development Hypothesis requires. Nor is its value lessened by the fact that many of the details and intermediate stages seem passed over in the development of the higher forms, for the recapitulation can only be of outlines, not of details; since there are differences in the forms, there must be differences in their histories.
107. In the preceding observations the object has simply been to show that the phenomena to be explained can be rationally conceived as resulting from gradual Evolution, whereas they cannot be so rationally interpreted on any other hypothesis. And here it may be needful to say a word respecting Epigenesis.
The Preformation hypothesis, which regarded every organism as a simple educt and not the product of a germ, was called by its advocates an evolution hypothesis—meaning that the adult form was an outgrowth of the germ, the miniature magnified. Wolff, who replaced that conception by a truer one, called his, by contrast, Epigenesis, meaning that there was not simplyout-growth butnewgrowth. “The various parts,” he says, “arise oneafterthe other, so that always one is secreted from (excernirt), or deposited (deponirt) on the other; and then it is either a free and independent part, or is only fixed to that which gave it existence, or else is contained within it. So thatevery part is the effect of a pre-existing part, and inturn the cause of a succeeding part.”51The last sentence expresses the conception of Epigenesis which embryologists now adopt; and having said this, we may admit that Wolff, in combating the error of preformation, replacing it with the truer notion of gradual and successive formation, was occasionally open to the criticism made by Von Baer, that he missed the true sense of Evolution, since the new parts are notadded onto the old parts as new formations, butevolved fromthem as transformations. “The word Evolution, therefore, seems to me more descriptive of the process than Epigenesis. It is true that the organism is not preformed, but the course of its development is precisely the course which its parents formerly passed through. Thus it is the Invisible—the course of development—which is predetermined.”52When the word Epigenesis is used, therefore, the reader will understand it to signify that necessary succession which determines the existence of new forms. Just as the formation of chalk is not the indifferent product of any combination of its elements, carbon, oxygen, and calcium, but is the product of only one series of combinations, an evolution through necessary successions, the carbon uniting with oxygen to form carbonic acid, and this combining with the oxide of calcium to form chalk, so likewise the formation of a muscle, a bone, a limb, or a joint has its successive stages, each of which is necessary, none of which can be transposed. The formation of bone is peculiarly instructive, because the large proportion of inorganic matter in its substance, and seemingly deposited in the organic tissue, would lead one to suppose that it was almost an accidental formation, which might take place anywhere; yetalthough what is called connective tissue will ossify under certain conditions, true bone is the product of a very peculiar modification, which almost always needs to be preceded by cartilage. That the formation of bone has its special history may be seen in the fact that it is the last to appear in the animal series, many highly organized fishes being without it, and all the other systems appearing before it in the development of the embryo. Thus although the mother’s blood furnishes all the requisite material, the fœtus is incapable of assimilating this material and of forming bone, until its own development has reached a certain stage. Moreover, when ossification does begin, it generally begins in the skull (in man in the clavicle); and the only approach to an internal skeleton in the Invertebrates is the so-called skull of the Cephalopoda. Not only is bone a late development, but cartilage is also; and although it is an error to maintain that the Invertebrates are wholly destitute of cartilage, its occasional presence having been fully proved by Claparède and Gegenbaur, the rarity of its presence is very significant. The animals which can form shells of chalk and chitine are yet incapable of forming even an approach to bone.
108. Epigenesis depends on the laws of succession, which may be likened to the laws of crystallization, if we bear in mind the essential differences between a crystal and an organism, the latter retaining its individuality through an incessant molecular change, the former only by the exclusion of all change. When a crystalline solution takes shape, it will always take a definite shape, which represents what may be called thedirectionof its forces, the polarity of its constituent molecules. In like manner, when an organic plasmode takes shape—crystallizes, so to speak—it always assumes a specific shape dependent on the polarity of its molecules. Crystallographershave determined the several forms possible to crystals; histologists have recorded the several forms of Organites, Tissues, and Organs. Owing to the greater variety in elementary composition, there is in organic substance a more various polar distribution than in crystals; nevertheless, there are sharply defined limits never overstepped, and these constitute what may be called the specific forms of Organites, Tissues, Organs, Organisms. An epithelial cell, for example, may be ciliated or columnar, a muscle-fibre striated or non-striated, a nerve-fibre naked or enveloped in a sheath, but the kind is always sharply defined. An intestinal tube may be a uniform canal, or a canal differentiated into several unlike compartments, with several unlike glandular appendages. A spinal column may be a uniform solid axis, or a highly diversified segmented axis. A limb may be an arm, or a leg, a wing, or a paddle. In every case the anatomist recognizes a specific type. He assigns the uniformities to the uniformity of the substance thus variously shaped, under a history which has been similar; the diversities he assigns to the various conditions under which the processes of growth have been determined. He never expects a muscular tissue to develop into a skeleton, a nervous tissue into a gland, an osseous tissue into a sensory organ. He never expects a tail to become a hand or a foot, though he sees it in monkeys and marsupials serving the offices of prehension and locomotion. He never expects to find fingers growing anywhere except from metacarpal bones, or an arm developed from a skull. The well-known generalization of Geoffroy St. Hilaire that an organ is more easily annihilated than transposed, points to the fundamental law of Epigenesis. In the same direction point all the facts of growth. Out of a formless germinal membrane we see an immense variety of forms evolved; and out of a common nutritive fluid this varietyof organs is sustained, repaired, replaced; and this not indifferently, not casually, but according to rigorous laws of succession; that which precedes determining that which succeeds as inevitably as youth precedes maturity, and maturity decay. The nourishment of various organs from plasmodes derived from a common fluid, each selecting from that fluid only those molecules that are like its own, rejecting all the rest, is very similar to the formation of various crystals in a solution of different salts, each salt separating from the solution only those molecules that are like itself. Reil long ago called attention to this analogy. He observed that if in a solution of nitre and sulphate of soda a crystal of nitre be dropped, all the dissolved nitre crystallizes, the sulphate remaining in solution; whereas on reversing the experiment, a crystal of sulphate of soda is found to crystallize all the dissolved sulphate, leaving the nitre undisturbed. In like manner muscle selects from the blood its own materials which are there in solution, rejecting those which the nerve will select.
109. Nay, so definite is the course of growth, that when a limb or part of a limb is cut off from a crab or salamander, a new limb or new part is reproduced in the old spot, exactly like the one removed. Bonnet startled the world by the announcement that theNaïs, a worm common in ponds, spontaneously divided itself into two worms; and that when he cut it into several pieces, each piece reproduced head and tail, and grew into a perfect worm. This had been accepted by all naturalists without demur, until Dr. Williams, in his “Report on British Annelida, 1851,” declared it to be a fable. In 1858, under the impulse of Dr. Williams’s very emphatic denial, I repeated experiments similar to those of Bonnet, with similar results. I cut two worms in half, and threw away the head-bearing segments, placing the others in two separate vessels, withnothing but water and a little mud, which was first carefully inspected to see that no worm lay concealed therein. In a few days the heads were completely reformed, and I had the pleasure of watching them during their reconstruction. When the worms were quite perfect, I again cut away their heads, and again saw these reformed. This was repeated, till I had seen four heads reproduced; after which the worms succumbed.
110. The question naturally arises, Why does the nutritive fluid furnish only material which is formed into a part like the old one, instead of reproducing another part, or one having a somewhat different structure? The answer to this question is the key to the chief problem of organic life. That a limbin situshould replace its molecular waste by molecules derived from the blood, seems intelligible enough (because we are familiar with it), and may be likened to the formation of crystals in a solution; but how is it that the limbwhich is not in existencecan assimilate materials from the blood? How is it that the blood, which elsewhere in the organism will form other parts, here will only form this particular part? There is, probably, no one who has turned his attention to these subjects who has not paused to consider this mystery. The most accredited answer at present before the world is one so metaphysiological that I should pass it by, were it not intimately allied with that conception of Species, which it is the object of these pages to root out. It is this:
111. The organism is determined by its Type, or, as the Germans say, its Idea. All its parts take shape according to this ruling plan; consequently, when any part is removed, it is reproduced according to the Idea of the whole of which it forms a part. Milne Edwards, in a very interesting and suggestive work, concludes his survey of organic phenomena in these words: “Dans l’organismetout semble calculé en vue d’un résultat déterminé, et l’harmonie des parties ne résulte pas de l’influence qu’elles peuvent exercer les unes sur les autres, mais de leur co-ordination sous l’empire d’une puissance commune, d’un plan préconçu, d’une force pré-existante.”53This is eminently metaphysiological. It refuses to acknowledge the operation of immanent properties, refuses to admit that the harmony of a complex structure results from the mutual relations of its parts, and seeksoutsidethe organism for some mysterious force, some plan, not otherwise specified, which regulates and shapes the parts. Von Baer, in his great work, has a section entitled, “The nature of the animal determines its development”; and he thus explains himself: “Although every stage in development is only made possible by the pre-existing condition [which is another mode of expressing Epigenesis], nevertheless the entire development is ruled and guided by the Nature of the animal which is about to be (von der gesammten Wesenheit des Thieres welches werden soll), and it is not the momentary condition which alone and absolutely determines the future, but more general and higher relations.”54One must always be slow in rejecting the thoughts of a master, and feel sure that one sees the source of the error before regarding it as an error; but in the present case I think the positive biologist will be at no loss to assign Von Baer’s error to its metaphysical origin. Without pausing here to accumulate examples both of anomalies and slighter deviations which are demonstrably due to the “momentary conditions” that preceded them, let us simply note the logical inconsistency of a position which, while assuming thatevery separate stagein development is the necessary sequence of its predecessor, declares thewhole of the stagesindependentof such relations! Such a position is indeed reconcilable on the assumption that animal forms are moulded “like clay in the hands of the potter.” But this is a theological dogma, which leads to very preposterous and impious conclusions; and whether it leads to these conclusions or to others, positive Biology declines theological explanations altogether. Von Baer, although he held the doctrine of Epigenesis, coupled it, as many others have done, with metaphysical doctrines to which it is radically opposed. He believed in Types as realities; he was therefore consistent in saying, “It is not the Matter and its arrangements which determine the product, but the nature of the parent form—the Idea, according to the new school.” How are we to understand this Idea? If it mean an independent Entity, an agency external to the organism, we refuse to acknowledge its existence. If it mean only ana posterioriabstraction expressing the totality of the conditions, then, indeed, we acknowledge that it determines the animal form; but this is only an abbreviated way of expressing the law of Evolution, by which each stage determines its successor. The Type does notdominatethe conditions, itemergesfrom them; the animal organism is not cast in a mould, but the imaginary mould is the form which the polarities of the organic substance assume. It would seem very absurd to suppose that crystals assumed their definite shapes (when the liquid which held their molecules in solution is evaporated) under the determining impulse of phantom-crystals, or Ideas; yet it has not been thought absurd to assume phantom forms of organisms.
112. The conception of Type as a determining influence arises from that fallacy of taking a resultant for a principle, which has played so conspicuous a part in the history of philosophy. Like many others of its class it exhibits an interesting evolution from the crude metaphysicalto the subtle metaphysical point of view, which at last insensibly blends into the positive point of view. At first the Type or Idea was regarded as an objective reality, external to the organism it was supposed to rule. Then this notion was replaced by an approach to the more rational interpretation, the idea was made an internal not an external force, and was incorporated with the material elements of the organism, which were said to “endeavor” to arrange themselves according to the Type. Thus Treviranus declares that the seed “dreams of the future flower”; and “Henle, when he affirms that hair and nails grow in virtue of the Idea, is forced to add that the parts endeavor to arrange themselves according to this Idea.”55Even Lotze, who has argued so victoriously against the vitalists, and has made it clear that an organism is a vital mechanism, cannot relinquish this conception of legislative Ideas, though he significantly adds, “these have no power in themselves, but only in as far as they are grounded in mechanical conditions.” Why then superfluously add them to the conditions? If every part of a watch, in virtue of the properties inherent in its substance, and of the mutual reactions of these and other parts, has a mechanical value, and if the sum of all these parts is the time-indicating mechanism, do we add to our knowledge of the watch, and our means of repairing or improving it, by assuming that the parts have over and above their physical properties the metaphysical “tendency” or “desire” to arrange themselves into this specific form? When we see that an organism is constructed of various parts, each of which has its own properties inalienable from its structure, and its uses dependent on its relation to other parts, do we gain any larger insight by crediting these parts with desires or“dreams” of a future result which their union will effect? That which is true in this conception of legislative Ideas is that when the parts come together there is mutual reaction, and the resultant of the whole is something very unlike the mere addition of the items, just as water is very unlike oxygen or hydrogen; further, the connexus of the whole impresses a peculiar direction on the development of the parts, and the law of Epigenesis necessitates a serial development, which may easily be interpreted as due to a preordained plan.
113. In a word, this conception of Type only adds a new name to the old difficulty, adding mist to darkness. The law of Epigenesis, which is simply the expression of the material process determined by the polarity of molecules, explains as much of the phenomena as is explicable. A lost limb is replaced by the very processes, and through the same progressive stages as those which originally produced it. We have a demonstration of its not being reformed according to any Idea or Type which exists apart from the immanent properties of the organic molecules, in the fact that it is not reformed at once, but by gradual evolution; the mass of cells at the stump are cells of embryonic character, cells such as those which originally “crystallized” into muscles, nerves, vessels, and integument, and each cell passes through all its ordinary stages of development. It is to be remembered that so intimately dependent is the result on the determining conditions, that any external influence which disturbs the normal course of development will either produce an anomaly, or frustrate the formation of a new limb altogether. One of my tritons bit off the leg of his female;56the leg which replaced it was much malformed,and curled over the back so as to be useless; was this according to the Idea? I cut it off, and examined it; all the bones were present, but the humerus was twisted, and of small size. In a few weeks a new leg was developed, and this leg was normal. If the Idea, as a ruling power, determined the growth of this third leg, what determined the second, which was malformed? Are we to suppose that in normal growth the Idea prevails, in abnormal the conditions? That it is the polarity of the molecules which at each moment determines the group those molecules will assume, is well seen in the experiment of Lavalle mentioned by Bronn.57He showed that if when an octohedral crystal is forming, an angle be cut away, so as to produce an artificial surface, a similar surface is produced spontaneously on the corresponding angle, whereas all the other angles are sharply defined. “Valentin,” says Mr. Darwin, “injured the caudal extremity of an embryo, and three days afterwards it produced rudiments of a double pelvis, and of double hind limbs. Hunter and others have observed lizards with their tails reproduced and doubled. When Bonnet divided longitudinally the foot of the salamander, several additional digits were occasionally formed.”58Where is the evidence of the Idea in these cases?
114. I repeat, the reproduction of lost limbs is due to a process which is in all essential respects the same as that which originally produced them; the genesis of one group of cells is the necessary condition for the genesis of its successor, nor can this order be transposed. But—and the point is very important—it is not every part that can be reproduced, nor is it every animal that has reproductive powers. The worm, or the mollusk, seems capable of reproducing every part; the crab will reproduce its claws, but not its head or tail; the perfect insect of the higher orders will reproduce no part (indeed the amputation of its antennae only is fatal), the salamander will reproduce its leg, the frog not. In human beings a muscle is said never to be reproduced; but this is not the case in the rare examples of supplementary fingers and toes, which have been known to grow again after amputation. The explanation of this difference in the reproductive powers of different animals is usually assigned to the degree in which their organisms retain the embryonic condition; and this explanation is made plausible by the fact that the animals which when adult have no power of replacing lost limbs, have the power when in the larval state. But although this may in some cases be the true explanation, there are many in which it fails, as will be acknowledged after a survey of the extremely various organisms at widely different parts of the animal series which possess the reproductive power. Even animals in the same class, and at the same stage of development, differ in this respect. I do not attach much importance to the fact that all my experiments on marine annelids failed to furnish evidence of their power of reproducing lost segments; because it is difficult to keep them under conditions similar to those in which they live. But it issignificant that, among the hundreds which have passed under my observation, not one should have been found with a head-segment in the process of development, replacing one that had been destroyed; and this is all the more remarkable from the great tenacity of life which the mutilated segments manifest. Quatrefages had observed portions of a worm, after gangrene had destroyed its head and several segments, move about in the water and avoid the light!59
115. A final argument to show that the reproduction is not determined by any ruling Idea, but by the organic conditions and the necessary stages of evolution, is seen in the reappearance of a tumor or cancer after it has been removed. We find the new tissue appear with all the characters of the normal tissue of the gland, then rapidly assume one by one the characters of the diseased tissue which had been removed; and there as on is, that the regeneration of the tissue is accompanied by the same abnormal conditions which formerly gave rise to the tumor: the directions of “crystallization” are similar because the conditions are similar. In every case of growth or regrowth the conditions being the same, the result must be the same.
116. It seems a truism to insist that similarity in the results must be due to similarity in the conditions; yet it is one which many theorists disregard; and especially do we need to bear it in mind when arguing about Species. I will here only touch on the suggestive topic of the analogies observed not simply among animals at the extreme ends of the scale, but also between animals and plants where the idea of a direct kinship is out of the question.
My very imperfect zoölogical knowledge will not allow me to adduce a long array of instances, but such an array will assuredly occur to every well-stored mind. It is enough to point to the many analogies of Function, more especially in the reproductive processes—to the existence of burrowers, waders, flyers, swimmers in various classes—to the existence of predatory mammals, predatory birds, predatory reptiles, predatory insects by the side of herbivorous congeners,—to the nest-building and incubating fishes; and in the matter of Structure the analogies are even more illustrative when we consider the widely diffused spicula, setæ, spines, hooks, tentacles, beaks, feathery forms, nettling-organs, poison-sacs, luminous organs, etc.; because these have the obvious impress of being due to a community of substance under similar conditions rather than to a community of kinship. The beak of the tadpole, the cephalopod, the male salmon, and the bird, are no doubt in many respects unlike; but there is a significant likeness among them, which constitutes a true analogy. I think there is such an analogy between the air-bladder of fishes and the tracheal rudiment which is found in the gnat-larva (Corethra plumicornis).60Veryremarkable also is the resemblance of theavicularium, or “bird’s-head process,” on the polyzoon known popularly as the Corkscrew Coralline (Bugula avicularia), which presents us in miniature with a vulture’s head—two mandibles, one fixed, the other moved by muscles visible within the head. No one can watch this organ snapping incessantly, without being reminded of a vulture, yet no one would suppose for a moment that the resemblance has anything to do with kinship.
117. Such cases are commonly robbed of their due significance by being dismissed as coincidences. But what determines the coincidence? If we assume, as we are justified in assuming, that thepossible directionsof Organic Combination, and the resultant forms, are limited, there must inevitably occur such coincident lines: the hooks on a Climbing Plant will resemble the hooks on a Crustacean or the claws of a Bird, as the one form in which under similar external forces the more solid but not massive portions of the integument tend to develop. I am too ill acquainted with the anatomy of plants to say how the hooks so common among them arise; but from examination of the Blackberry, and comparison of its thorns with the hooks and spines of the Crustacea, I am led to infer that in each case the mode of development is identical—namely, the secretion of chitine from the cellular matrix of the integument.
Another mode of evading the real significance of such resemblances is to call them analogies, not homologies. There is an advantage in having two such terms, but we ought to be very clear as to their meaning and their point of separation. Analogy is used to designate similarity in Function with dissimilarity in Structure. The wing of an insect, the wing of a bird, and the wing of a bat are called analogous, but not homologous, because their anatomical structure is different: they are not constructed out of similar anatomical parts. The fore-leg of a mammal, the wing of a bird, or the paddle of a whale, are called homologous, because in spite of their diverse uses they are constructed out of corresponding anatomical parts. To the anatomist such distinctions are eminently serviceable. But they have led to some misconceptions, because they are connected with a profound misconception of the relation between Function and Organ. Embryology teaches that the wing of the bird and the paddle of the whale are developed out of corresponding parts, and that these are not like the parts from which the wing of an insect or the flying-fish will be developed; nevertheless, the most cursory inspection reveals that the wing of a bird and the paddle of a whale are very unlike in structure no less than in function, and that their diversities in function correspond with their diversities in structure; whereas the wing of the insect, of the bird, and of the bat, are in certain characters very similar, and correspondingly there are similarities in their function. It is, however, obvious that the resemblance in function is strictly limited to the resemblance in anatomical structure; only in loose ordinary speech can the flight of an insect, a bird, or a bat be said to be “the same”: it is different in each—the weight to be moved, the rapidity of the movement, the precision of the movements, and their endurance, all differ.
118. It is impossible to treat of Evolution without taking notice of that luminous hypothesis by which Mr. Darwin has revolutionized Zoölogy. There are two points needful to be clearly apprehended before the question is entered upon. The first point relates to the lax use of the phrase “conditions,” sometimes more instructively replaced by “conditions of existence.” Inasmuch as Life is only possible under definite relations of the organism and its medium, the “conditions of existence” will be those physical, chemical, and physiological changes, whichinthe organism, andoutof it,co-operateto produce the result. There are myriads of changes in the external medium which have no corresponding changes in the organism, not being in any direct relation to it (see §54). These, not being co-operant conditions, must be left out of the account; they are not conditions of existence for the organism, and therefore the organism does not vary with their variations. On the other hand, what seem very slight changes in the medium are often responded to by important changes in the vital chemistry, and consequently in the structure of the organism. Now the nature of the organism at the time being, that is to say, its structure and the physico-chemical state of its tissues and plasmodes, is the mainconditionof this response; the same external agent will be powerful, or powerless, over slightly different organisms, or over the same organism at different times. Usually, and for convenience, when biologists speak of conditions, they only refer to external changes. This usage has been the source of no little confusion in discussing the Development Hypothesis. Mr. Darwin, however, while following the established usage, is careful in several places to declare that of the two factors in Variation—the nature of the organismand the nature of the conditions—the former is by far the more important.
118a. A still greater modification of terms must now be made. Instead of confining the “struggle for existence” to the competition of rivals and the antagonism of foes, we must extend it to the competition and antagonism of tissues and organs. The existence of an organism is not only dependent on the external existence of others, and is the outcome of a struggle; but also on the internal conditions which co-operate in the formation of its structure, this structure being the outcome of a struggle. The organism is thisparticularorganism, differing from others, because of the particular conditions which have co-operated. The primary and fundamental struggle must be that of the organic forces at work in creating a structure capable of pushing its way amid external forces. The organism must find a footing in the world, before it can compete with rivals, and defend itself against foes. Owing to the power of reproduction, every organism has a potential indefiniteness of multiplication; that potential indefiniteness is, however, in reality restricted by the supply of food, and by the competition of rivals for that supply. The multiplication of any one species is thus kept down by the presence of rivals and foes: a balance is reached, which permits of the restricted quantities of various species. This balance is the result of a struggle.
Now let me call attention to a similar process in the formation of the organism itself. Every organite, and every tissue, has a potential growth of indefinite extent, but its real growth is rigorously limited by the competition and antagonism of the others, each of which has its potential indefiniteness, and its real limits. Something, in the food assimilated, slightly alters the part which assimilates it. This change may be the origin ofother changes in the part itself, or in neighboring parts, stimulating or arresting the vital processes. A modification of structure results. Or there may be no new substance assimilated, but external forces may call a part into increased activity—which means increased waste and repair; and this increase here is the cause of a corresponding decrease somewhere else. Whatever the nature of the change, it finds its place amid a complex of changes, and its results are compounded with theirs. When organites and tissues are said to have a potential indefiniteness of growth, there is assumed a potential indefiniteness in the pabulum supplied:ifthe pabulum were supplied, andifthere were no antagonism thwarting its assimilation, growth would of course continue without pause, or end; but in reality this cannot be so. For, take the blood as the vehicle of the pabulum—not only is its quantity limited, and partly limited by the very action of the tissues it feeds, but even in any given quantity there is a limit to its composition—it will only take up a limited quantity of salts, iron, albumen, etc.; no matter how abundant these may be in the food. So again with the plasmodes of the various tissues—they have each their definite capacities of assimilation. What has already been stated respecting chemical affinity (§20) is equally applicable to organic affinity; as the presence of fused iron in the crucible partially obstructs the combination of sulphur and lead, so the presence of connective tissue partially obstructs the combination of muscle protoplasm with its pabulum.
118b. Owing to the action and reaction of blood and plasmode, of tissues on tissues, and organs on organs, and their mutual limitations, the growth of each organism has a limit, and the growth of each organ has a limit. Beyond this limit, no extra supply of food will increase the size of the organism; no increase of activity willincrease the organ. “Man cannot add a cubit to his stature.” The blacksmith’s arm will not grow larger by twenty years of daily exercise, after it has once attained a certain size. Increase of activity caused it to enlarge up to this limit; but no increase of activity will cause it to pass this limit. Why? Because here a balance of the co-operating formative forces has been reached. Larger muscles, or more muscle-fibres, demand arteries of larger calibre, and these a heart of larger size; with the increase of muscle would come increase of connective tissue; and this tissue would not only compete with the muscle for pabulum, but by mechanical pressure would diminish the flow of that pabulum. And why would connective tissue increase? Because, in the first place, there is a formative association between the two, so that owing to a law, not yet understood, the one always accompanies the other; and, in the second place, there is a functional association between the two, a muscle-fibre beinginoperativeunless it be attached to a tendon, or connective tissue; it will contractoutof the body although separated from its tendon or other attachment; butinthe body its contraction would be useless without this attachment. We must bear in mind that muscle-fibres are very much shorter than ordinary muscles; according to the measurements of W. Krause they never exceed 4cmin length, and usually range between 2 and 3cm; their fine points being fixed to the interstitial connective tissue, as the whole muscle is fixed to its tendon. The function of the muscle is thus dependent on a due balance of its component tissues; if that balance is disturbed the function is disturbed. Should, from any cause, an excess of muscle-fibre arise, the balance would be disturbed; should an encroachment of connective tissue, or of fat, take place, there would be also a defect of function.
Here we have the co-operation and limitation of thetissues illustrated; let us extend our glance, and we shall see how the co-operation and limitation of the organs come into play, so that the resulting function depends on the balance of their forces. The contractile power of each individual muscle is always limited by the resistance of antagonists, which prevent the muscle being contracted more than about a third of itspossibleextent, i. e. possible when there are no resistances to be overcome. Not only the increasing tension of antagonist muscles, but the resistance of tendons, bones, and softer parts must be taken into account. Thus, the increase of the blacksmith’s muscular power would involve a considerable increase in all the tissues of the arm; but such an increase would involve a reconstruction of his whole organism.
Whenever there is an encroachment of one tissue on another, there is a disturbance of the normal balance, which readily passes into a pathological state. If the brain is overrun with connective tissue, or the heart with fatty tissue, we know the consequences. If connective tissue is deficient, epithelial runs to excess, no longer limited by its normal antagonist, and pus, or cancer, result.
118c. It is unnecessary here to enlarge on this point. I have adduced it to show that we must extend our conception of the struggle for existence beyond that of the competition and antagonism of organisms—the external struggle; and include under it the competition and antagonism of tissues and organs—the internal struggle. Variability is inherent in organic substances, as the result of their indefiniteness of composition (§45b). This variability is indefinite, and is rendered definite by the competition and antagonism, so that every particular variation is the resultant of a composition of forces. The forces in operation are the internal and external conditions of existence—i. e. the nature of the organism, and its response to the actions of its medium. A change may take placein the medium without a corresponding response from the organism; or the change may find a response and the organism become modified. Every modification is a selection, determined by laws of growth; it is the resultant of a struggle between what, for want of a better term, may be called theorganic affinities—which represent in organized substances what chemical affinities are in the anorganized. Just as an organism which has been modified and thereby gained a superiority over others, has by this modification beenselectedfor survival—the selection being only another aspect of this modification—so one tissue, or one organ, which has surpassed another in the struggle of growth, will thereby have become selected. Natural Selection, or survival of the fittest, therefore, is simply the metaphorical expression of the fact that any balance of the forces which is best adapted for survival will survive. Unless we interpret it as a shorthand expression ofallthe internal and external conditions of existence, it is not acceptable as the origin of species.
118d. Mr. Darwin has so patiently and profoundly meditated on the whole subject, that we must be very slow in presuming him to have overlooked any important point. I know that he has not altogether overlooked this which we are now considering; but he is so preoccupied with the tracing out of his splendid discovery in all its bearings, that he has thrown the emphasis mainly on the external struggle, neglecting the internal struggle; and has thus in many passages employed language which implies a radical distinction where—as I conceive—no such distinction can be recognized. “Natural Selection,” he says, “depends on the survival under various and complex circumstances of the best-fitted individuals, but has no relation whatever to the primary cause of any modification of structure.”61On this we may remark, first, thatselection does notdependon the survival, butisthat survival; secondly, that the best-fitted individual survives because of that modification of its structure which has given it the superiority; therefore if the primary cause of this modification is not due to selection, then selection cannot be the cause of species. He separates Natural Selection from all the primary causes of variation, either internal or external—either as results of the laws of growth, of the correlations of variation, of use and disuse, etc., and limits it to the slow accumulations of such variations as are profitable in the struggle with competitors. And for his purpose this separation is necessary. But biological philosophy must, I think, regard the distinction as artificial, referring only to one of the great factors in the production of species. And for this reason: Selection only comes into existence in the modifications produced either by external or internal changes; and the selected change cannot be developed further by mere inheritance, unless the successive progeny have such a disposition of the organic affinities as will repeat the primary change. Inherited superiority will not by mere transmission become greater. The facts which are relied on in support of the idea of “fixity of species” show at any rate that a given superiority will remain stationary for thousands of years; and no one supposes that the progeny of an organism will vary unless some external or internal cause of variation accompanies the inheritance. Mr. Darwin agrees with Mr. Spencer in admitting the difficulty of distinguishing between the effects of some definite action of external conditions, and the accumulation through natural selection of inherited variations serviceable to the organism. But even in cases where the distinction could be clearly established, I think we should only see anhistoricaldistinction, that is to say, one between effects produced by particular causes nowin operation, and effects produced by very complex and obscure causes in operation during ancestral development.
118e. The reader will understand that my criticism does not pretend to invalidate Mr. Darwin’s discovery, but rather to enlarge its terms, so as to make it include all the biological conditions, and thus explain many of the variations which Natural Selection—in the restricted acceptation—leaves out of account. Mr. Darwin draws a broad line of distinction between Variation and Selection, regarding only those variations that are favorable as selected. I conceive that all variations which survive are by that fact of survival,selections, whether favorable or indifferent. A variety is a species in formation; now Selection itself is not a cause, or condition, of variation, it is theexpressionof variation. Mr. Darwin is at times explicit enough on this head: “It may metaphorically be said that Natural Selection is daily and hourly scrutinizing throughout the world the slightest variations; rejecting those that are bad, preserving and adding up all that are good; silently and insensibly working, whenever and wherever opportunity offers, at the improvement of each organic being in relation to its organic and inorganic conditions of life.”62But the metaphorical nature of the term is not always borne in mind, so that elsewhere Natural Selection is said to “act on and modify organic beings,” as if it were a positive condition and not the expression of the modifying processes. Because grouse are largely destroyed by birds of prey, any change in their color which would render them less conspicuous would enable more birds to escape; but it is obvious that this change of color will be due to Organic Affinity; and only when the change iseffectedwill there have been that selection whichexpressesit. Mr. Darwin’s language, however,is misleading. He says: “Hence Natural Selection might be most effective ingivingthe proper color to each kind of grouse, and inkeepingthat color when once acquired.” This is to make Selection an agent, a condition of the development of color; which may be accepted if we extend the term so as to include the organic changes themselves. Again: “Some writers have imagined that Natural Selectioninduces variability, whereas it only implies thepreservationof such variations as are beneficial to the being under its conditions of life.” It, however, is made to imply more than this, namely, the accumulation and further modification of such variations. “The mere existence of individual variability and of some well-marked varieties, though necessary as the foundation, helps us but little in understanding how species arise in nature. How have all those exquisite adaptations of one part of the organization to another part, and to the conditions of life, and of one organic being to another being, been perfected?” My answer to this question would be: By Organic Affinity, and the resulting struggle of the tissues and organs, the consequences of which are that veryadaptationof the organism to external conditions, which is expressed as theselectionof the structures best adapted. The selections are the results of the struggle, according to my proposed extension of the term “struggle.” Mr. Darwin defines the struggle: “The dependence of one being on another, and including (what is more important) not only the life of the individual but success in leaving progeny.” This definition seems defective, since it omits the primary and more important struggle which takes place between the organic affinities in operation. To succeed in the struggle with competitors, the organism must have first acquired—by selection—a superiority in one or more of its organs.
118f. A little reflection will disclose the importanceof keeping our eyes fixed on the internal causes of variation, as well as on the external conditions of the struggle. Mr. Darwin seems to imply that the external conditions which cause a variation are to be distinguished from the conditions which accumulate and perfect such variation, that is to say, he implies a radical difference between the process of variation and the process of selection. This, I have already said, does not seem to me acceptable; the selection, I conceive, to be simply the variation which has survived.63
If it be true that a Variety is an incipient Species and shows us Species in formation, it is in the same sense true that a variation is an incipient organ. A species is the result of a slowly accumulating divergence of structure; an organ is the result of a slowly accumulating differentiation. At each stage of differentiation there has been a selection, but we cannot by any means say that this selection was determined by the fact of its giving the organism a superiority over rivals, inasmuch as during all the early stages, while the organ was still in formation, there could be no advantage accruing from it. One animal having teeth and claws developed will have a decided superiority in the struggle over another animal that has no teeth and claws; but so long as the teeth and claws are in an undeveloped state of mere preparation they confer no superiority.
118g. Natural Selection is only the expression of theresults of obscure physiological processes; and for a satisfactory theory of such results we must understand the nature of the processes. In other words, to understand Natural Selection we must recognize not only the facts thus expressed, but the factors of these facts,—we must analyze the “conditions of existence.” As a preliminary analysis we findexternal conditions, among which are included not only the dependence of the organism on the inorganic medium, but also the dependence of one organism on another,—the competition and antagonism of the whole organic world; andinternal conditions, among which are included not only the dependence of the organism on the laws of composition and decomposition whereby each organite and each tissue is formed, but also the dependence of one organite and one tissue on all the others—the competition and antagonism of all the elements.
The changes wrought in an organism by these two kinds of conditions determine Varieties and Species. Although many of the changes are due to the process of natural selection brought about in the struggle with competitors and foes, many other changes have no such relation to the external struggle, but are simply the results of the organic affinities. They may or they may not give the organism a greater stability, or a greater advantage over rivals; it is enough that they are no disadvantage to the organism, they will then survive by virtue of the forces which produced them.
119. The position thus reached will be important in our examination of the Theory of Descent by which Mr. Darwin tentatively, and his followers boldly, explain the observed resemblances in structure and function as due to blood-relationship. The doctrine of Evolution affirms that all complex organisms are evolved by differentiation from simpler organisms, as we see the complex organ evolved from simpler forms. But it does not necessarily affirmthat the vast variety of organisms had one starting-point—one ancestor; on the contrary, I conceive that the principles of Evolution are adverse to such a view, and insist rather on the necessity of innumerable starting-points. Let us consider the question.
That the Theory of Descent explains many of the facts must be admitted; but there are many which it leaves obscure; and Mr. Darwin, with that noble calmness which distinguishes him, admits the numerous difficulties. Whether these will hereafter be cleared away by an improvement in the Geological Record, now confessedly imperfect, or by more exhaustive exploration of distant countries, none can say; but, to my mind, the probability is, that we shall have to seek our explanation by enlarging the idea of Natural Selection, subordinating it to the laws of Organic Affinity. It does not seem to me, at present, warrantable to assume Descent as the sole principle of morphological uniformities; there are other grounds of resemblance beyond those of blood-relationship; and these have been too much overlooked; yet a brief consideration will disclose that similarity in the physiological laws and the conditions of Organic Affinity must produce similarity in organisms, independently of relationship; just as similarity in the laws and conditions of inorganic affinity will produce identity in chemical species. We do not suppose the carbonates and phosphates found in various parts of the globe, or the families of alkaloids and salts, to have any nearer kinship than that which consists in the similarity of their elements and the conditions of their combination. Hence, in organisms, as in salts, morphological identity may be due to a community of conditions, rather than community of descent. Mr. Darwin justly holds it to be “incredible that individuals identically the same should have been produced through Natural Selection from parentsspecificallydistinct,” but he, since he admits analogous variations, will not deny that identical forms might issue from parents having widely different origins, provided that these parent forms and the conditions of their reproduction were identical, as in the case of vegetable and animal resemblances. To deny this would be to deny the law of causation. And that which is true of identical forms under identical conditions is true of similar forms under similar conditions. When History and Ethnology reveal a striking uniformity in the progression of social phases, we do not thence conclude that the nations are directly related, or that the social forms have a common parentage; we conclude that the social phases are alike because they have had common causes. When chemists point out the uniformity of type which exists in compounds so diverse in many of their properties as water and sulphuretted or selenetted hydrogen, and when they declare phosphoretted hydrogen to be the congener of ammonia, they do not mean that the one is descended from the other, or that any closer link connects them than that of resemblance in their elements.
In the case of vegetal and animal organisms, we observe such a community of elementary substance as of itself to imply a community in their laws of combination; and under similar conditions the evolved forms must be similar. With this community of elementary substance, there are also diversities of substance and of co-operant conditions; corresponding with these diversities there must be differences of form. Thus, although observation reveals that the bond of kinship does really unite many widely divergent forms, and the principle of Descent with Natural Selection will account for many of the resemblances and differences, there is at present no warrant for assuming that all resemblances and differences are due to this one cause, but, on the contrary, weare justified in assuming a deeper principle which may be thus formulated: All the complex organisms are evolved from organisms less complex, as these were evolved from simpler forms; the link which unites all organisms is not always the common bond of heritage, but the uniformity of organized substance acting under similar conditions.
It is therefore consistent with the hypothesis of Evolution to admit a variety of origins or starting-points, though not consistent to admit the sudden appearance of complex Types, such as is implied in the hypothesis of specific creations.
119a. The analogies of organic forms and functions demand a more exhaustive scrutiny than has yet been given them. Why is it that vessels, nerves, and bonesramifylike branches, and why do these branches take on the aspect of many crystalline forms? Why is it that cavities are constantly prolonged in ducts, e. g. the mouth succeeded by the œsophagus, the stomach by the intestines, the bladder by the urethra, the heart by the aorta, the ovary by the oviduct, and so on? Why are there never more than four limbs attached to a vertebral column, and these always attached to particular vertebræ? Why is there a tendency in certain tissues to form tubes, and in these tubes commonly to assume a muscular coat?64To some of these queries an answer might be suggested which would bring them under known physical laws. I merely notice them here for the sake of emphasizing the fact that such analogies lie deeply imbedded in the laws of evolution, and that what has been metaphorically called organic crystallization will account for many similarities in form, without forcing us to have recourse to kinship. To take a very simple case. No one will maintain that the crystalline forms of snow have any kinship with the plants which they often resemble.Mr. Spencer has noticed the development of a wing-bearing branch from a wing of thePtilota plumosa, when its nutrition is in excess. “This form, so strikingly like that of the feathery crystallizations of many inorganic substances, proves to us that in such crystallizations the simplicity or complexity of structure at any place depends on the quantity of matter that has to be polarized at that place in a given time. How the element of time modifies the result, is shown by the familiar fact that crystals rapidly formed are small, and that they become larger when they are formed more slowly.”65
It may be objected, and justly, that in the resemblance between crystals and organisms the analogy is purely that of form, and usually confined to one element, whereas between organisms there is resemblance of substance no less than of form, and usually the organisms are alike in several respects. The answer to this objection is, that wherever there is a similarity in the causal conditions (substance and history) there must be a corresponding similarity in the results; if this similarity extends to only a few of the conditions, the analogy will be slight; if to several, deep. But whether slight or deep we are not justified, simply on the ground of resemblance, in assuming, short of evidence, that because they are alike, two organisms are related by descent from a common ancestor.