"The causes of definitely directed evolution are contained, according to my view, in the effects produced by outward circumstances and influences such as climate and nutrition upon the constitution of a given organism."
"At variance with all the facts of definitely directed evolution ... is also the contention of my opponent [Weismann] ... that the variations demonstrably oscillate to and fro in the most diverse directions about a given zero-point. There is no oscillation in the direction of development, but simply an advance forwards in a straight line with occasional lateral divergences whereby the forkings of the ancestral tree are produced."[[59]]
These sentences contain one of those explanations which explain nothing; for we are not enabled to see how the "outward circumstances and influences" produce the effects ascribed to them. We are not shown in what way they cause organic evolution in general, still less in what way they cause the infinitely-varied forms in which organic evolution results. The assertion that evolution takes definitely-directed lines is accompanied by no indication of the reasons why particular lines are followed rather than others. In short, we are simply taken a step back, and for further interpretation referred to a cause said to be adequate, but the operations of which we are to imagine as best we may.
This is a re-introduction of supernaturalism under a disguise. It may pair off with the conception made popular by the Vestiges of the Natural History of Creation, in which it was contended that there exists a persistent tendency towards the birth of a higher form of creature; or it may be bracketed with the idea entertained by the late Prof. Owen, who alleged an "ordained becoming" of living things.
§ 174d. An objection to the Darwinian doctrine which has risen into prominence, is that Natural Selection does not explain that which it professes to explain. In the words of Mr. J. T. Cunningham:—
"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.