The adaptation of animals and plants to the conditions under which they live has always excited the interest, and also the imagination, of philosophers and scientists; for this relation between the organism and its environment is one of the most characteristic features of living things. The question at once suggests itself: How has such a relation been brought about? Is it due to something inherent in the living matter itself, or is it something that has been, as it were, superimposed upon it? An example may make my meaning clearer. No one will suppose that there is anything inherent in iron and other metals that would cause them to produce an engine if left to themselves. The particular arrangement of the pieces has been superimposed upon the metals, so that they now fulfil a purpose, or use. Have the materials of which organisms are composed been given a definite arrangement, so that they fulfil the purpose of maintaining the existence of the organism; and if so, how has this been accomplished? It is the object of the following pages to discuss this question in all its bearings, and to give, as far as possible, an idea of the present state of biological thought concerning the problem. I trust that the reader will not be disappointed if he finds in the sequel that many of the most fundamental questions in regard to adaptation are still unsettled.

In attempting to state the problem as clearly as possible, I fear that it may appear that at times I have “taken sides,” when I should only have been justified in stating the different aspects of the question. But this will do little harm provided the issue has been sharply drawn. Indeed, it seems to me that the only scientific value, that a discussion of what the French call “les grands problÈmes de la Biologie” has, is to get a clearer understanding of the relation of what is known to what is unknown or only surmised.

In some quarters speculation concerning the origin of the adaptation of living things is frowned upon, but I have failed to observe that the critics themselves refrain entirely from theorizing. They shut one door only to open another, which also leads out into the dark. To deny the right to speculative thought would be to deny the right to use one of the best tools of research.

Yet it must be admitted that all speculation is not equally valuable. The advance of science in the last hundred years has shown that the kind of speculation that has real worth is that which leads the way to further research and possible discovery. Speculation that leads to this end must be recognized as legitimate. It becomes useless when it deals with problems that cannot be put to the actual test of observation or experiment. It is in this spirit that I have approached the topics discussed in the following pages.

The unsophisticated man believes that all other animals exist to minister to his welfare; and from this point of view their adaptations are thought of solely in their relation to himself. A step in advance was taken when the idea was conceived that adaptations are for the good of the organisms themselves. It seemed a further advance when the conclusion was reached that the origin of adaptations could be accounted for, as the result of the benefit that they conferred on their possessor. This view was the outcome of the acceptation of the theory of evolution, combined with Darwin’s theory of natural selection. It is the view held by most biologists at the present time; but I venture to prophesy that if any one will undertake to question modern zoologists and botanists concerning their relation to the Darwinian theory, he will find that, while professing in a general way to hold this theory, most biologists have many reservations and doubts, which they either keep to themselves or, at any rate, do not allow to interfere either with their teaching of the Darwinian doctrine or with the applications that they may make of it in their writings. The claim of the opponents of the theory that Darwinism has become a dogma contains more truth than the nominal followers of this school find pleasant to hear; but let us not, therefore, too hastily conclude that Darwin’s theory is without value in relation to one side of the problem of adaptation; for, while we can profitably reject, as I believe, much of the theory of natural selection, and more especially the idea that adaptations have arisen because of their usefulness, yet the fact that living things must be adapted more or less well to their environment in order to remain in existence may, after all, account for the widespread occurrence of adaptation in animals and plants. It is this point of view that will be developed in the following pages.

I am fully aware of the danger in attempting to cover so wide a field as that of “Evolution and Adaptation,” and I cannot hope to escape the criticism that is certain to be directed against a specialist who ventures nowadays beyond the immediate field of his own researches; yet, in my own defence, I may state that the whole point of view underlying the position here taken is the immediate outcome of my work on regeneration. One of the general questions that I have always kept before me in my study of regenerative phenomena is how such a useful acquirement as the power to replace lost parts has arisen, and whether the Darwinian hypothesis is adequate to explain the result. The conclusion that I have reached is that the theory is entirely inadequate to account for the origin of the power to regenerate; and it seemed to me, therefore, desirable to reËxamine the whole question of adaptation, for might it not prove true here, also, that the theory of natural selection was inapplicable? This was my starting-point. The results of my examination are given in the following pages.

I am deeply indebted to Professor G. H. Parker and to Professor E. G. Conklin for advice and friendly criticism; and in connection with the revision of the proof I am under many obligations to Professor Joseph W. Warren and to Professor E. A. Andrews. Without their generous help I should scarcely have ventured into a field so full of pitfalls.

Bryn Mawr, Penn., June 10, 1903.