These three facts--namely, the limited duration of individual life, the uninterrupted succession of generations, and the differences of the existing animals and plants from those of former geological periods whose remains are preserved in stratified rocks--are sufficient by themselves to prove that evolution has taken place, that the history of organisms has been a process of descent with modification. If the animals and plants whose remains are preserved as fossils, or at any rate forms closely related to these, were not the ancestors of existing forms, there are only two other possibilities: either the existing forms came into existence by new creations after the older forms became extinct, or the ancestors of existing forms, although they coexisted with the older forms, never left any fossil remains. Each of these suppositions is incredible. In view of these plain facts and their logical conclusion it is curious to notice how Darwin in his _Origin of Species_ constantly mingles together arguments to prove the proposition that evolution has occurred, that the structure and relations of existing animals can only be explained by descent with modification, with arguments and evidence in favour of natural selection as the explanation and cause of evolution. In the great controversy about evolution which his work aroused, the majority of the educated public were ultimately convinced of the truth of evolution by the belief that a sufficient cause of the process of change had been discovered, rather than by the logical conclusion that the organisms of a later period were the descendants of those of earlier periods. Even at the present day the theory of natural selection is constantly confused with the doctrine of evolution. The fact is that the investigation of the causes of evolution has been going on and has been making progress from the time of Darwin, and from times much earlier than his, down to the present day. Bionomics show that every type must be adapted in structure to maintain its life under the conditions in which it lives, the primary requirements being food and oxygen. Every animal must be able to procure food either of various kinds or some special kind--either plants or other animals; it may be adapted to feed on plants or to catch insects or fish or animals similar to itself; its digestive organs must be adapted to the kind of food it takes; it must have respiratory organs adapted to breathe in air or water; it must produce eggs able to survive in particular conditions, and so on.