ANIMAL ECOLOGY The aspects of zoology which we have so far considered are morphological in their outlook; in them it is form and not function which is the centre of interest. Even the development of genetics has led to the explanation of observed differences of structure by an inferred structure in chromosomes. But from the very begin ning of zoology a different point of view has been adopted by some investigators. To the common man the activities of an animal are far more interesting than its structure, and professional zoologists, after an interval of half a century, are recognizing the justification for his belief. Thus a steadily increasing, though still small proportion of zoological research is devoted to physiology (q.v.), to the study of animal function. Such studies may, as in the recent remarkable developments of muscle physiology, be carried out without any reference to observed structural parts, but sooner or later, and in most cases from their beginning, they have to be brought into relation with morphology.
Animals are alive, and they live in a world presenting a wide range of variation of conditions, both inorganic and organic. The study of the relations between an animal and its environment, which is called bionomics or ecology (q.v.), is that proper to the field naturalist ; it must be based on observation of natural con ditions, interpreted by experiments designed to eliminate or evaluate the many factors which are involved. Such studies were first made an integral part of the science of zoology by Darwin. It is curious and unfortunate, though understandable, that whilst the reactions of Darwin's work on morphology and taxonomy were immediate and salutary, those studies which formed the main part of his life work were neglected by professed zoologists in general, and their further developments rapidly became fantastic because little or no attempt was made to test the validity of the innumer able hypothetical explanations of "observed" facts which were enunciated.
It has always been known that the distribution of animals over the world is not uniform; every schoolboy is aware that kangaroos are found only in Australia, and that the sea coast is inhabited by many birds which are seldom or never found inland. Study of
this irregular distribution can be carried out in two ways; the fauna of some one natural area may be compared with that of others, or the detailed distribution of the animals within a small area may be discussed.
The results of the first type of investigation are fully treated in the article ZOOLOGICAL REGIONS. They are of interest from the light that they shed on the course of evolution, especially in regard to the migrations of animals and the existence of evolutionary centres. The value of these conclusions depends entirely on the extent to which the taxonomic divisions which are recognized, and the relationship of animals which are implied in them, are true representations of fact.
It is apparent that the existing distribution of animals cannot be accounted for on a basis of the geography of to-day, and a study of zoo-geographical regions necessarily leads the student into the perilous fields of palaeogeography. The results reached by an investigation of the distribution of the larger groups of animals must be shown to be consistent with one another; and with the distribution of land, water and mountain chains in past time which may be inferred from geological evidence. The only method of controlling the accuracy of the conclusions which are reached lies in palaeontological evidence, which gives definite in formation, even although partial, about the distrihiition of many animals in the various periods of geological time.
Bionomics, or ecology (q.v.), is a study of a very different kind, still largely undeveloped, but capable of becoming of the highest importance, not only to theoretical zoology, but to those who are engaged in the commercial exploitation of animals and plants, and to all men who are exposed to the risk of infection by animal borne diseases.