It is obvious that every animal is so far fitted for life that it is capable of maintaining itself under certain conditions. The field naturalists of the period before Darwin had observed that many structures of animals, even if very grotesque, played a definite part in their lives, and they reached the conclusion that every part and peculiarity of an animal had a definite function, being designed by the Creator to render service to the animal in that state of life in which it lived. This belief was at once seized on by theologians, as evidence of the existence of a beneficent Creator. Lamarck and Darwin both accepted the zoological aspect of these views, and the latter brought to their support a vast mass of observations drawn either from his own world-wide observations or from the reports of other writers. (See EVOLUTION ; ADAPTATION.) But both Lamarck and Darwin rejected the idea that these "adapta tions," whereby an animal was specially fitted for life in a definite environmental niche, were the result of deliberate design; each put forward a theory which purported to explain how they could here arise under the action of intelligible forces during the course of evolution.
The Lamarckian explanation, which has already been referred to, requires an inheritance of acquired characters of a kind which cannot be proved to occur, and has never met with any general acceptance.
Darwin's theory of natural selection provides, at the least, a complete formal explanation of all, or nearly all, evolution. But it still rests on an insecure basis. Darwin was the first zoolo gist to realize that an incredibly small proportion of all the animals which are born on the globe survive to maturity. The world popu lation of herrings has probably been sensibly constant for a very long period, although each individual female herring lays many millions of eggs during its lifetime. Of these millions only a few individuals reach maturity and breed in their turn. The remainder die at some early stage, most of them serving as food for other animals. Darwin enquired whether the animals which survived were in any way different from those which perished ; appealing to the facts of variation, which he collected for the first time, he claimed that it is obvious that, in general, the survivors will owe their escape to the fact that they were better fitted in some way or ways for life under the conditions to which they were subjected. That this view expresses a general truth can scarcely be doubted, hut it is unsatisfactory that only very few (about 17) attempts have ever been made to establish its truth in individual cases, and to measure the intensity of the selection which is so exercised.
Having established the existence of a natural selection, Darwin proceeded to discuss whether or not the variations which secured the survival of the selected individuals would be inherited by their offspring. He appealed to the experience of breeders of domesti cated animals, and thus, for the first time, brought the study of heredity into zoological science.
The evidence of breeders, imperfect though it was, justified Darwin in claiming that the favourable variations of the animals spared by natural selection would be passed on to their descend ants. If, then, these in turn varied and new favourable varia tions were selected, the process would be repeated, with the result that after many generations the stock under consideration would have changed so as to become much better adapted to the condi tions under which it lived. The knowledge of heredity which we now possess shows nothing which invalidates Darwin's postulates, and his doctrine is still available.
Darwin and his followers applied this conception to the explana tion of a vast series of curious structures in animals and plants, and introduced subsidiary hypotheses such as that of sexual selec tion (q.v.) to explain special groups of cases. But only in very few of these many cases was any attempt made to show by direct observation and experiment that a structure did, in fact, fulfil the function for which it was supposed to be adapted. In many cases the suggested explanation became so far fetched as to be gro tesque, with the result that a number of living zoologists have come to disbelieve in the existence of adaptation as an important phenomenon, and to have a contempt for those who still discuss it. Nevertheless, the existence of such evolutionary series as those of the horse and many others shows that a steady improve ment of the mechanism of limbs has taken place as an adaptation to some special type of locomotion. And no zoologist who has any extended acquaintance with animals under their natural condi tions can doubt that each is, in fact, very well fitted for the life it leads. Of late years the pendulum of opinion has been swinging back towards an essentially Darwinian position.
It is clear that any study of the relations existing between an animal and its environment must begin with an evaluation of the properties of the latter. Many circumstances make it more easy to express the conditions of a body of water, and especially of the sea, in figures, than those of a land area, and it is hence not surprising that the science of oceanography (q.v.—see also FISH ERIES) was founded and has been developed by zoologists.