"Everybody knows that the theory of natural selection was put forward by Darwin as a theory of the origin of species, and yet it is only a theory of the origin of adaptations. The question is: Are the differences between species differences of adaptation? If so, then the origin of species and the origin of adaptations are equivalent terms. But there is scarcely a single instance in which a specific character has been shown to be useful, to be adaptive."60
To illustrate this last statement Mr. Cunningham names the plaice, flounder, and dab as three flat fishes in which, along with the adaptive characters related to the mode of life common to them all, each has specific characters which are not adaptive. No evidence is forthcoming that these in any way conduce to the welfare of the species. Two propositions are here involved which should be separately dealt with.
The first is that the adaptive modifications which survival of the fittest is able to produce, do not become specific traits: they are traits separate in kind from those which mark off groups proved to be specifically distinct by their inability to breed together. Such evidence as we at present have seems to warrant this statement. Out of the many varieties of dogs most, if not all, have been rendered distinct by adaptive modifications, mostly produced by selection. But, notwithstanding the immense divergences of structure so produced, the varieties inter-breed. To this, however, it may be replied that sufficient time has not elapsed – that the process by which a structural adaptation so reacts on the constitution as to make it a distinct one, possibly, or probably, takes many thousands of years. Let us accept for the moment Lord Kelvin's low estimate of the geologic time during which life has existed – one hundred million years. Suppose we divide that time into as many parts as there are hours occupied in the development of a human fœtus. And suppose that during these hundred million years there has been going on with some uniformity the evolution of the various organic types now existing. Then the amount of change undergone by the fœtus in an hour, will be equivalent to the amount of change undergone by an evolving organic form in fifteen thousand years. That is to say, during general evolution it may have taken fifteen thousand years to establish, as distinct, two species differing from one another no more than the fœtus differs from itself after the lapse of an hour. Hence, though we lack proof that adaptive modifications become specific traits, it is quite possible that they are in course of becoming specific traits.
The converse proposition, that the traits by which species are ordinarily distinguished are non-adaptive traits is well sustained; and the statement that, if not themselves useful they are correlated with those which are useful, is, to say the least, unproved. For the instances given by Mr. Darwin of correlated traits are not those between adaptive traits and the traits regarded as specific, but between traits none of which are specific; as between skull and limbs in swine, tusks and bristles in swine, horns and wool in sheep, beak and feet in pigeons.
If we seek a clue in those processes by which correlations are brought about – the physiological actions and reactions – we may at once see that any organic modification, be it adaptive or not, must entail secondary modifications throughout the rest of the organism, most of them insensible but some of them sensible. The competition for blood among organs, referred to above, necessitates that, other things remaining the same, the extra growth of any one tells on all others, in variable degrees according to conditions, and may cause appreciable diminutions of some. This is not all. While the quantity of blood supplied to other organs is affected, its quality also is in some cases affected. Each organ, or at any rate each class of organs, has special nutrition – abstracts from the blood a proportion of ingredients different from that abstracted by other organs or classes of organs. Hence may result a deficiency or a surplus of some element: instance the change in the blood which must be caused by growth of a stag's antlers. Now if such effects are always produced, and if, further, a change of general nutrition caused by a new food or by a difference of ability to utilize certain components of food, similarly operates (instance the above named correlation between horns and wool), then every modification must entail throughout the organism multitudinous alterations of structure. Such alterations will ordinarily be neither in themselves useful nor necessarily correlated with those which are useful; since they must arise as concomitants of any change, whether adaptive or not. There will consequently arise the innumerable minute differences presented by allied species in addition to the differences called specific.
On joining with recognition of this general process a recognition of the tendency towards localization of deposit, one possible origin of specific marks is suggested. When in an organism the circulating fluids contain useless matter, normal or abnormal, the excretion of it, once determined towards a certain place, continues at that place. Trees furnish examples in the casting out of gums and resins. Animal life yields evidence in gouty concretions and such morbid products as tubercle. A place of enfeebled nutrition is commonly chosen – not unfrequently a place where a local injury has occurred. Now if we extend this principle, well recognized in pathological processes, to physiological processes, we may infer that where an adaptive modification has so reacted on the blood as to leave some matter to be got rid of, the deposit of this, initiated at some place of least resistance, may produce a local structure which eventually becomes a species-mark. A relevant inquiry suggests itself – What proportion of species-marks are formed out of inanimate tissue or tissue of low vitality – tissue which, like hair, feathers, horns, teeth, is composed of by-products unfit for carrying on vital actions.
§ 174e. In the days when, not having been better instructed by Mr. Darwin, I believed that all changes of structure in organisms result from changes of function, I held that the cause of such changes of function is migration. Assuming as the antecedent of migration a great geologic change, such as upheaval of the East Indian Archipelago step by step into a continent, it was argued, in an essay I then wrote, that, subjected primarily to new influences in its original habitat, each kind of plant and animal would secondarily be subjected to the altered conditions consequent on spreading over the upheaved regions.
"Each species being distributed over an area of some extent, and tending continually to colonize the new area exposed, its different members would be subject to different sets of changes. Plants and animals spreading towards the equator would not be affected in the same way with others spreading from it. Those spreading towards the new shores would undergo changes unlike the changes undergone by those spreading into the mountains. Thus, each original race of organisms would become the root from which diverged several races differing more or less from it and from one another."
It was further argued that, beyond modifications caused by change of physical conditions and food, others would be caused by contact of the Flora and Fauna of each island with the Floras and Faunas of other islands: bringing experience of animals and plants before unknown.61
While this conception was wrong in so far as it ascribed the production of new species entirely to inheritance of functionally-wrought alterations (thus failing to recognize Natural Selection, which was not yet enunciated), it was right in so far as it ascribed organic changes to changes of conditions. And it was, I think, also right in so far as it implied that isolation is a condition precedent to such changes. Apparently it did not occur to me as needful to specify this isolation as making possible the differentiation of species; since it goes without saying that members of a species spreading east, west, north, south, and forming groups hundreds of miles apart, must, while breeding with those of the same group be prevented from breeding with those of other groups – prevented from having their locally-caused modifications mutually cancelled.
The importance of isolation has of late been emphasized by Dr. Romanes and others, who, to that isolation consequent on geographical diffusion, have added that isolation which results from difference of station in the same habitat, and also that due to differences in the breeding periods arising in members of the same species. Doubtless in whatever way effected, the isolation of a group subject to new conditions and in course of being changed, is requisite as a means to permanent differentiation. Doubtless also, as contended by Mr. Gulick and Dr. Romanes, there is a difference between the case in which an entire species being subject to the same conditions is throughout modified in character, thus illustrating what Mr. Gulick calls "monotypic evolution," and the case in which different parts of the species, leading different lives, will, if they are by any means prevented from inter-breeding with other parts, form divergent varieties: thus illustrating "polytypic evolution."
§ 174f. Beyond geographical and topographical isolation, there is an isolation of another kind regarded by some as having had an important share in organic evolution. Foreshadowed by Mr. Belt, subsequently enunciated by Mr. Catchpool, fully thought out by Mr. Gulick, and more recently elaborated by Dr. Romanes, "Physiological Selection" is held to account for the genesis of marked varieties side by side with their parents. It is contended that without the kind of isolation implied by it, variations will be swamped by inter-crossing, and divergence prevented; but that by the aid of this kind of isolation, a uniform species may be differentiated into two or more species, though its members continue to live in the same area.
Facts are assigned to show that slightly unlike varieties may become unable to inter-breed either with the parent-species or with one another. This mutual inferiority is not of the kind we might expect. We might reasonably suppose that when varieties had diverged widely, crossing would be impracticable, because their constitutions had become so far unlike as to form an unworkable mixture. But there seems evidence that the infertility arises long before such a cause could operate, and that instead of failure to produce a workable constitution, there is failure to produce any constitution at all – failure to fertilize. Some change in the sexual system is suggested as accounting for this. That a minute difference in the reproductive elements may suffice, plants prove by the fact that when two members of slightly-divergent varieties are fertilized by each other's pollen, the fertility is less than if each were fertilized by the pollen of its own variety; and where the two kinds of pollen are both used, that derived from members of the same variety is prepotent in its effect over that derived from members of the other variety.
The writers above named contend that variations must occur in the reproductive organs as well as in other organs; that such variations may produce relative infertility in particular directions; and that such relative infertility may be the first step towards prevention of crossing and establishment of isolation: so making possible the accumulation of such differences as mark off new species. Without doubt we have here a legitimate supposition and a legitimate inference. Necessarily there must happen variations of the kind alleged, and considering how sensitive the reproductive system is to occult influences (witness among ourselves the frequent infertility of healthy people while feeble unhealthy ones are fertile), it is reasonable to infer that minute and obscure alterations of this kind may make slightly-different varieties unable to inter-breed.
Granting that there goes on this "physiological selection," we must recognize it as one among the causes by which isolation is produced, and the differentiating influence of natural selection in the same locality made possible.
§ 174g. The foregoing criticisms and hypotheses do not, however, affect in any essential way the pre-existing conceptions. If, as in the foregoing chapters, we interpret the facts in terms of that redistribution of matter and motion constituting Evolution at large, we shall see that the general theory, as previously held, remains outstanding.
It is indisputable that to maintain its life an organism must maintain the moving equilibrium of its functions in presence of environing actions. This is a truism: overthrow of the equilibrium is death. It is a corollary that when the environment is changed, the equilibrium of functions is disturbed, and there must follow one of two results – either the equilibrium is overthrown or it is re-adjusted: there is a re-equilibration. Only two possible ways of effecting the re-adjustment exist – the direct and the indirect. In the one case the changed outer action so alters the moving equilibrium as to call forth an equivalent reaction which balances it. If re-equilibration is not thus effected in the individual it is effected in the succession of individuals. Either the species altogether disappears, or else there disappear, generation after generation, those members of it the equilibria of whose functions are least congruous with the changed actions in the environment; and this is the survival of the fittest or natural selection.
If now we persist in thus contemplating the problem as a statico-dynamical one, we shall see that much of the discussion commonly carried on is beside the question. The centre around which the collision of arguments has taken place, is the question of the formation of species. But here we see that this question is a secondary and, in a sense, irrelevant one. We are concerned with the production of evolving and diverging organic forms; and whether these are or are not marked off by so-called specific traits, and whether they will or will not breed together, matters little to the general argument. If two divisions of a species, falling into unlike conditions and becoming re-equilibrated with them, eventually acquire the differences of nature called specific, this is but a collateral result. The essential result is the formation of divergent organic forms. The biologic atmosphere, so to speak, has been vitiated by the conceptions of past naturalists, with whom the identification and classification of species was the be-all and end-all of their science, and who regarded the traits which enabled them to mark off their specimens from one another, as the traits of cardinal importance in Nature. But after ignoring these technical ideas it becomes manifest that the distinctions, morphological or physiological, taken as tests of species, are merely incidental phenomena.
Moreover, on continuing thus to look at the facts, we shall better understand the relation between adaptive and specific characters, and between specific characters and those many small differences which always accompany them. For during re-equilibration there must, beyond those changes of structure required to balance outer actions by inner actions, be numerous minor changes. In any complex moving equilibrium alterations of larger elements inevitably cause alterations of elements immediately dependent on them, and these again of others: the effects reverberate and re-reverberate throughout the entire aggregate of actions down to the most minute. Of resulting structural changes a few will be conspicuous, more will be less conspicuous, and so on continuously multiplying in number and decreasing in amount.
Here seems a fit place for remarking that there are certain processes which do not enter into these re-equilibrations but in a sense interfere with them. One example must suffice. Among dogs may be observed the trick of rolling on some mass having a strong animal smell: commonly a decaying carcase. This trick has probably been derived from the trick of rolling on the body of an animal caught and killed, and so gaining a tempting odour. A male dog which first did this, and left a trail apt to be mistaken for that of prey, would be more easily found by a female, and would be more likely than others to leave posterity. Now such a trick could have no relation to better maintenance of the moving equilibrium, and might very well arise in a dog having no superiority. If it arose in one of the worst it would be eliminated from the species, but if it arose in one of medium constitution, fairly capable of self-preservation, it would tend to produce survival of certain of the less fit rather than the fittest. Probably there are many such minor traits which are in a sense accidental, and are neither adaptive nor specific in the ordinary sense.
§ 174h. But now let it be confessed that though all phenomena of organic evolution must fall within the lines above indicated, there remain many unsolved problems.
Take as an instance the descent of the testes in the Mammalia. Neither direct nor indirect equilibration accounts for this. We cannot consider it an adaptive change, since there seems no way in which the production of sperm-cells, internally carried on in a bird, is made external by adjustment to the changed requirements of mammalian life. Nor can we ascribe it to survival of the fittest; for it is incredible that any mammal was ever advantaged in the struggle for life by this changed position of these organs. Contrariwise, the removal of them from a place of safety to a place of danger, would seem to be negatived by natural selection. Nor can we regard the transposition as a concomitant of re-equilibration; since it can hardly be due to some change in the general physiological balance.
An example of another order is furnished by the mason-wasp. Several instincts, capacities, peculiarities, which are in a sense independent though they cooperate to the same end, are here displayed. There is the instinct to build a cell of grains of sand, and the ability to do this, which though in a sense separate may be regarded as an accompaniment; and there is the secretion of a cement – a physiological process not directly connected with the psychological process. After oviposition there comes into play the instinct to seek, carry home, and pack into the cell, the small caterpillars, spiders, &c., which are to serve as food for the larva; and then there is the instinct to sting each of them at a spot where the injected hypnotic poison keeps the creature insensible though alive till it is wanted. These cannot be regarded as parts of a whole developed in simultaneous coordination. There is no direct connexion between the building instinct and the hypnotizing instinct; still less between these instincts and the associated appliances. What were the early stages they passed through imagination fails to suggest. Their usefulness depends on their combination; and this combination would seem to have been useless until they had all reached something like their present completeness. Nor can we in this case ascribe anything to the influence of teaching by imitation, supposed to explain the doings of social insects; for the mason-wasp is solitary.
Thus the process of organic evolution is far from being fully understood. We can only suppose that as there are devised by human beings many puzzles apparently unanswerable till the answer is given, and many necromantic tricks which seem impossible till the mode of performance is shown; so there are apparently incomprehensible results which are really achieved by natural processes. Or, otherwise, we must conclude that since Life itself proves to be in its ultimate nature inconceivable, there is probably an inconceivable element in its ultimate workings.