INTRODUCTION 3 



as being physiology. There is another aspect of the subject, however, 

 which is not less fundamental, but of a different nature. The processes 

 of development in embryos are included in this branch of zoology. The 

 events which initiate the division of an egg, control the folding of cell 

 layers, direct the migration of cells, and govern differential rates of growth 

 in a developing embryo are physiological processes. Embryology thus 

 has a physiological aspect, as well as a purely morphological one. 



Physiology is also a very old branch of zoology although the subject 

 remained in a crude state long after morphology, particularly the mor- 

 phology of the higher mammals including man, had become fairly 

 definite. 



Ecology. — This branch of zoology has to do with the relations of 

 animals to their environment. These relations may be manifold. The 

 necessity for getting food puts an animal into relation to the things 

 about it. Its method of locomotion is intimately related to the medium 

 in which it lives. The presence of enemies and competitors is an almost 

 universal condition of life. The possibihties of rearing young are im- 

 portant in every animal's environment. The very behavior of an 

 organism especially in lower forms of life, as is pointed out in a later 

 chapter, is dependent upon the nature of the surroundings, so that a 

 minute change in the environment may completely reverse the organ- 

 ism's behavior. 



Very often the study of ecology takes the form of determining the 

 factors which govern an animal's choice of haMtat, or place of abode. 

 Animals of any species are found only or chiefly in certain types of en- 

 vironment, depending partly upon their structure and partly upon their 

 physiological requirements. Often, also, their occurrence in a given 

 environment is dependent on the presence of certain other animals. The 

 factors which induce, or force, an organism to seek an aerial, aquatic, 

 terrestrial, or subterranean abode are often quite obscure. One animal 

 must have a pond, rather than a stream, and not all ponds are equally 

 good ; nor is every part of the same pond a suitable place. The reasons 

 for these requirements are partly to be found in the animal, partly in 

 the surroundings; or, more correctly stated, in the relations existing 

 between the factors within the animal and those external to it. Thus, 

 ecology involves much that is morphology and physiology, but only in 

 so far as structure and function relate the organism to other things about 

 it. It is the work of the ecologist to discover the structure and functions 

 of an animal which are of significance in such relations, the features 

 of the environment to which they are related, and the nature of the 

 relation between them. Although every hunter, trapper or collector of 

 animals has long been a practical ecologist, scientific work of this kind has 

 only in comparatively recent years seriously occupied the minds of any 

 considerable number of zoologists. 



