Abstract: THE proper study of any aspect of zoology involves the consideration of the following facts: (1) the external world, in contradistinction to the organism, is undergoing constant and ceaseless change; (2) the organism itself is similarly subjected to change; (3) the organism and its environment are in a constant state of reaction. The physical sciences aim at a correct understanding of what is happening in matter in its innumerable aspects and the reality underlying these changes. The biologist seeks to elucidate the phenomena connected with the organism, but his study is never thorough unless an attempt is made to correlate the relation of the changing organism to the external world. It is not possible to explain correctly the form, shape or function of a particular organ, or the behaviour of a particular organism and its relationships to other organisms, without taking into account the third consideration mentioned above, in other words, that every organism is trying to fit itself to live in its particular surroundings. This effort on the part of the organism is, to a large extent, conditioning its structure, its behaviour and its life-processes.