24 PRINCIPLES OF ANIMAL BIOLOGY 



has been considered. It should not be forgotten that zoology has always 

 had its practical applications. In the last half century these applica- 

 tions have been greatly multiplied. Medicine is applied zoology, ap- 

 plied directly to man and incidentally to those animals that are parasitic 

 upon him or that are carriers of disease. Sanitation and public hygiene 

 are matters of apphed biology even more than matters of engineering. 

 Crop-destroying insects and other pests are combated by the economic 

 zoologist. Improvement of breeds of domestic animals is the practical 

 aim of the student of animal husbandry. The applications of zoology 

 to practical ends are still probably in their beginnings. Its applications 

 to man himself are particularly in a backward state. The improvement 

 of human qualities, the maintenance of the best human strains, the effects 

 of migration and intermarriage upon the races of men and their political 

 development, and the effects of war upon racial qualities, are problems 

 which still await solution. 



Although zoology has had its fashions, though it has been developed 

 in different directions at different times, these well-marked movements 

 do not all bear the same relation to each other. The announcement 

 of the cell theory stands in a pecuHarly significant position. Injtself 

 the discovery of cells was not more important than some of the other 

 developments of zoology. But it had and has unique possibilities as 

 the foundation of other branches of the science. Animals are made up 

 of cells, which is a purely morphological fact. But their physiology 

 is what it is because of the nature of cells. Inheritance and evolution 

 are cell phenomena. Relation to environment is dependent on the func- 

 tions of cells. Even geographical distribution and classification are 

 fundamentally dependent upon the cellular characteristics of animals. 

 As a preface, then, to the discussion of the various branches of zoology 

 in later chapters, a knowledge of cells is essential, and to that subject 

 attention is now turned. 



References 



LocT, W. A. Biology and Its Makers. 



OsBORN, H. F. From the Greeks to Darwin. 



MiALL, L. C. Tbe Early Naturalists, Tbeir Life and Work. 



