123. I come round to the position from which I started, that the resemblances traceable among animals are no proof of kinship; even a resemblance so close as to defy discrimination would not, in itself, be such a proof. The absolute identity of chalk in Australia and in Europe is a proof that there was absolute identity in the formative conditions and the constituent elements, but no proof whatever that the two substances were originally connected by genesis. In like manner the similarity of a plant or animal in Africa and Europe may be due to a common kinship, but it may also be due to a common history. It is indeed barely conceivable that the history, from first to last, would ever be so rigorously identical in two parts of the globe as to produce complex identical forms in both; because any diversity, either in structure or external conditions, may be the starting-point of a wide diversity in subsequent development; and the case of organic combinations is so far unlike the inorganic, that while only one form is possible to the latter (chalk is either formed or not formed), many forms are possible to organic elements owing to the complexity and indefiniteness of organic composition. But although forms so allied as those of Species are not readily assignable to an identical history in different quarters of the globe, it is not only conceivable, but is eminently probable, that Orders and Classes have no nearer link of relationship than is implied in their community of organized substance and their common history. The fact that there is not a single mammal common to Europe and Australia is explicable, as Mr. Darwin explains it, on the ground that migration has been impossible to them; but it is also explicable on the laws of Evolution—to have had mammals of the same species and genera would imply a minute coincidence in their history, which is against the probabilities. Again, in the Oceanic Islands there are no Batrachians; but there are Reptiles, and these conform to the reptilian type. Mr. Darwin suggests that the absence of Batrachia is due to the impossibility of migration, their ova being destroyed by salt water. But may it not be due to the divergence from the reptilian type, which was effected elsewhere, not having taken place in these regions? When we find the metal Tin in Prussia and Cornwall, and nowhere else in Europe, must we not conclude that in these two countries, and nowhere else, a peculiar conjunction of conditions caused this peculiar evolution?
124. The question at issue is, Are the resemblances observable among organic forms due to remote kinship, and their diversities to the divergences caused by adaptation to new conditions? or are the resemblances due to similarities, and the diversities to dissimilarities in the substance and history of organic beings? Are we to assume one starting-point and one centre of creation, or many similar starting-points at many centres? So far from believing that all plants and animals had their origin in one primordial cell, at one particular spot, from which descendants migrated and became diversified under the diverse conditions of their migration, it seems to me more consistent with the principle of Evolution to admit a vast variety of origins more or less resembling each other; and this initial resemblance will account for the similarities still traceable under the various forms; while the early differences, becoming intensified by development under different conditions, will yield the diversities. The evolution of organisms, like the evolution of crystals, or the evolution of islands and continents, is determined, 1st, by laws inherent in the substances evolved, and, 2d, by relations to the medium in which the evolution takes place. This being so, we may à priori affirm that the resultant forms will have a community strictly corresponding with the resemblance of the substances and their conditions of evolution, together with a diversity corresponding with their differences in substance and conditions. It is usually supposed that the admission of separate “centres of creation” is tantamount to an admission of “successive creations” as interpreted by the majority of those who invoke “creative fiats.” But the doctrine of Evolution, which regards Life as making its appearance consequent upon a concurrence of definite conditions, and regards the specific forms of Life as the necessary consequences of special circumstances, must also accept the probability of similar conditions occurring at different times and in different places. Upon what grounds, cosmical or biological, are we to assume that on only one microscopic spot of this developing planet such a group of conditions was found—on only one spot a particle of protein substance was formed out of the abundant elements, and under conditions which caused it to grow and multiply, till in time its descendants overran the globe? The hypothesis that all organic forms are the descendants of a single germ, or of even a few germs, and are therefore united by links of kinship more or less remote, is not more acceptable than the hypothesis that all the carbonates and phosphates, all the crystals, and all the strata found in different parts of the globe, are the descendants of a single molecule, or a few molecules; or,—since this may seem too extravagant,—than that the various maladies which afflict organic beings are, in a literal sense, members of families having a nearer relationship than that of being the phenomena manifested by similar organs under similar conditions—a conception which might have been accepted by those metaphysical pathologists who regarded Disease as an entity. Few philosophers have any hesitation in supposing that other planets besides our own are peopled with organic forms, though, from the great differences in the conditions, these forms must be extremely unlike those of our own planet. If separate worlds, why not separate centres? The conclusion seems inevitable that wherever and whenever the state of things permitted that peculiar combination of elements known as organized substance, there and then a centre was established—Life had a root. From roots closely resembling each other in all essential characters, but all more or less different, there have been developed the various stems of the great tree. Myriads of roots have probably perished without issue; myriads have developed into forms so ill-adapted to sustain the fluctuations of the medium, so ill-fitted for the struggle of existence, that they became extinct before even our organic record begins; myriads have become extinct since then; and the descendants of those which now survive are like the shattered regiments and companies after some terrific battle.
125. There seems to me only one alternative logically permissible to the Evolution Hypothesis, namely, that all organic forms have had either a single origin, or else numerous origins; in other words, that a primordial cell was the starting-point from which all organisms have been successively developed; or that the development issued from many independent starting-points, more or less varied. This is apparently not the aspect presented by the hypothesis to many of its advocates; they seem to consider that if all organic forms are not the lineal descendants of one progenitor, they must at any rate be the descendants of not more than four or five. The common belief inclines to one. Mr. Darwin, whose caution is as remarkable as his courage, and whose candor is delightful, hesitates as to which conclusion should be adopted: “I cannot doubt,” he says, “that the theory of descent, with modifications, embraces all the members of the same class. I believe that animals have descended from, at most, only four or five progenitors, and plants from an equal or lesser number. Analogy would lead me one step further, namely, to the belief that all animals and plants have descended from some one prototype. But analogy may be a deceitful guide.”
126. I cannot see the evidence which would warrant the belief that Life originated solely in one microscopic lump of protoplasm on one single point of our earth’s surface; on the contrary, it is more probable that from innumerable and separate points of this teeming earth, myriads of protoplast sprang into existence, whenever and wherever the conditions of the formation of organized substance were present. It is probable that this has been incessantly going on, and that every day new protoplasts appear, struggle for existence, and serve as food for more highly organized rivals; but whether an evolution of the lower forms is, or is not, still going on, there can be no reluctance on the part of every believer in Evolution to admit that when organized substance was first evolved, it was evolved at many points. If this be so, the community observable in organized substance, wherever found, may as often be due to the fact of a common elementary composition as to the fact of inheritance. If this be so, we have a simple explanation both of the fundamental resemblances which link all organisms together, and of the characteristic diversities which separate them into kingdoms, classes, and orders. The resemblances are many, and close, because the forms evolved had a similar elementary composition, and their stages of evolution were determined by similar conditions. The diversities are many, because the forms evolved had from the first some diversities in elementary composition, and their stages of evolution were determined under conditions which, though similar in general, have varied in particulars. Indeed, there is no other ground for the resemblances and differences among organic beings than the similarities and dissimilarities in their Substance and History; and, whether the similarities are due to blood-relationship, or to other causes, the results are the same. There is something seductive in the supposition that Life radiated from a single centre in ever-increasing circles, its forms becoming more and more various as they came under more various conditions, until at last the whole earth was crowded with diversified existences. “From one cell to myriads of complex organisms, through countless æons of development,” is a formula of speculative grandeur, but I cannot bring myself to accept it; and I think that a lingering influence of the tradition of a “creative fiat” may be traced in its conception. May we not rather assume that the earth at the dawn of Life was a vast germinal membrane, every slightly diversified point producing its own vital form; and these myriads upon myriads of forms—all alike and all unlike—urged by the indwelling tendencies of development, struggled with each other for existence, many failing, many victorious, the victors carrying their tents into the camping ground of the vanquished. The point raised is the immense improbability of organized substance having been evolved only in one microscopic spot; if it were evolved at more than one spot, and under slightly varying conditions, there would necessarily have arisen in these earliest formations the initial diversities which afterwards determined the essential independence and difference of organisms.
129. Let us for a moment glance at the resemblances and diversities observable in all organisms. All have a common basis, all being constructed out of the same fundamental elements: carbon, hydrogen, nitrogen, and oxygen; these (the organogens, as they are named), with varying additions of some other elements, make up what we know as Organic Substance, vegetal and animal. Another peculiarity all organisms have in common, namely, that their matter is neither solid nor liquid, but viscid. Beside this community of Substance we must now place a community of History. All organisms grow and multiply by the same process; all pass through metamorphic stages ending in death; all, except the very simplest, differentiate parts of their substance for special uses, and these parts (cilia, membranes, tubes, glands, muscles, nerves) have similar characters in whatever organism they appear, and their development is always similar, so that the muscles or nerves of an intestinal worm, a lobster, or a man, are in structure and history fundamentally alike. When, therefore, we see that there is no biological character of fundamental importance which is not universal throughout the organic world, when we see that in Structure and History all organisms have a community pervading every variety, it is difficult not to draw the conclusion that some hidden link connects all organisms into one; and when, further, it is seen that the most divergent forms may be so arranged by the help of intermediate forms only slightly varying one from the other, that the extreme ends—the monad and the man—may be connected, and a genealogical tree constructed, which will group all forms as modified descendants from a single form, the hypothesis that kinship is the bidden link of which we are in search becomes more and more cogent.
130. But now let the other aspect be considered. If there is an unmistakable uniformity, there is also a diversity no less unmistakable. The chemical composition of organic substances is various. Unlike inorganic substances, the composition of which is rigorously definite, organic substances are, within narrow limits, variable in composition (§ [45]).
I pass over the resemblances and differences observed in the earliest stages of development, marked as they are, and direct attention to the fact, that down at what must be considered the very lowest organic region, we meet with differences not less striking than those met with in the highest, we find structures (if structures they may be called), which cannot be affiliated, so widely divergent is their composition. The structureless vibrio, for example, is not only capable of living in a medium destitute of Oxygen, but is, according to M. Pasteur, actually killed by oxygen; whereas the equally simple bacteria can no more dispense with Oxygen than other animals can. Consider for a moment the differences implied in the fact that one organism cannot even form an enveloping membrane to contain its protoplasm, whereas another contrives to secrete an exquisite shell; yet between the naked Rhizopod and the shelled Rhizopod our lenses and reagents fail to detect a difference. One Monad can assimilate food of only one kind, another Monad assimilates various kinds.[73] What a revelation of chemical differences appears in the observations of M. Pasteur respecting the vibrio and bacteria, in a fermentescible liquid—the former beginning the putrid fermentation which the latter completes! We cannot doubt that some marked difference must exist between the single-celled organism which produces alcoholic fermentation, and that which produces acetic fermentation, and that again which produces butyric fermentation; and if we find distinctions thus established at the lowest region of the organic series, we need not marvel if the distinctions become wider and more numerous as the series becomes more diversified. The structure and development of an organism are dependent on the affinities of its constituent molecules, and it is a biological principle of great importance which Sir James Paget insists on, when he shows how “the existence of certain materials in the blood may determine the formation of structures in which they may be incorporated.”[74] Any initial diversity may thus become the starting-point of a considerable variation in subsequent evolution.[75] Thus, supposing that on a given spot there are a dozen protoplasts closely resembling each other, yet each in some one detail slightly varying; if this variation is one which, by its relations to the external medium, admits of a difference in the assimilation of materials present in the medium, it may be the origin of some new direction in development, and the ultimate consequence may be the formation of a shell, an internal skeleton, a muscle, or a nerve. Were this not so, it would be impossible to explain such facts as that chitine is peculiar to the Articulata, cellulose to Molluscoida, carbonates of lime to Mollusca and Crustacea, and phosphates to Vertebrata—all assimilated from the same external medium. But we see that from this medium one organism selects the materials which another rejects; and this selection is determined by the nature of the structure: which assimilates only those materials it is fitted to assimilate. We hear a great deal of Adaptation determining changes of structure and function, and are too apt to regard this process as if it were not intimately dependent on a corresponding structural change. By no amount of external influence which left the elementary composition of the structure unchanged, could an organism with only two tissues be developed into an organism with three or four. By no supply or stimulus, could an animal incapable of assimilating peroxide of iron acquire red blood corpuscles, although it might have the iron without the corpuscles; nor could an oyster form its shell unless capable of assimilating carbonate of lime. For myriads of years, in seas and ponds, under endless varieties of external conditions, the amœbæ have lived and died without forming a solid envelope, although the materials were abundant, and other organisms equally simple have formed envelopes of infinite variety. In all the seas, and from the earliest ages, zoophytes have lived, and assumed a marvellous variety of shapes and specialization of functions; but although some of them have acquired muscles, none have acquired true nerves, none bone. Ages upon ages rolled on before fishes were capable of forming bone; and thousands are still incapable of forming it, though living in the same waters as the osseous fishes.
131. “Looking to the dawn of life,” says Mr. Darwin (repeating an objection urged against his hypothesis), “when all organic beings, as we imagine, presented the simplest structure, how could the first steps in advancement, or in the differentiation and specialization of parts have arisen? I can make no sufficient answer; and can only say that, as we have no facts to guide us, all speculation would be baseless and useless.”
Where Mr. Darwin hesitates, lesser men need extra caution; but I must risk the danger of presumption, at least so far as to suggest that while an answer to this question is difficult on that dynamical view of Evolution which regards Function as determining Structure, it is less difficult on the statico-dynamical view propounded in these pages; the difficulty which besets the explanation when all the manifold varieties of organic forms are conceived as the successive divergences from an original starting-point, is lessened when a variety of different starting-points is assumed, in each of which some initial diversity prepared the way for subsequent differentiations; just as we know that between the ovum of a vertebrate and the ovum of an invertebrate, similar as they are, there is a diversity which manifests itself in their subsequent evolution. If Function is determined by Structure, and Evolution is the product of the two, it is clear that the different directions in the lines of development will have their origin in structural differences, and not in the action of external circumstances, unless these previously bring about a structural change. The action of the medium on the organism is assuredly a potent factor which Biology cannot ignore: but the organism itself is a factor, and according to its nature the influence of the medium is defined. (§ [118].)
132. Quitting for a moment the track of this argument, let us glance at the resemblances and differences observable in Plants and Animals, because most people admit that these have separate origins. The resemblances are scarcely less significant than those existing among animals. Both have a similar basis of elementary composition; not only are both formed out of protoplasts with similar properties, but in both the first step from the protoplasm to definite structure is the Cell. And the life of this Cell is remarkably alike in both, its phases of development being in many respects identical; nay, even such variations as obtain in the cell-membranes are curiously linked together by a community in the formative process.[76] In both Plants and Animals we find individuals constituted—1st, by single cells; 2d, by groups of cells undistinguishable among each other; and 3d, by groups of differentiated cells. In both we find colonies of individuals leading a common life. In both the processes of Nutrition and Reproduction are essentially similar; both propagate sexually and asexually; both exhibit the surprising phenomena of parthenogenesis and alternate generations. In both there are examples of a free-roving embryo which in maturity becomes fixed to one spot, losing its locomotive organs and developing its reproductive organs. In both the development of the reproductive organs is the climax which carries Death. So close is the analogy between plant-life and animal-life, that it even reaches the properties usually held to be exclusively animal; I mean that even should we hesitate to accept Cohn’s discovery of the muscles in certain plants,[77] we cannot deny that plants exhibit Contractility; and should we refuse to interpret as Sensibility the phenomena exhibited by the Sensitive Plants, we cannot deny that they present a very striking analogy to the phenomena of Sensibility exhibited by animals.