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.