DEFINITION OF ZOOLOGY. g 



the gradual development of each part and of the func- 

 tion which it performs to its completion in the mature 

 condition. This is embryology. 



Now, if there were but one kind of animal in the 

 world, the study of that species from these three points of 

 view would still be inexhaustible. But the field of study 

 becomes far wider when we remember that there are an 

 almost infinite variety of kinds, of every degree of com- 

 plexity of organization, and that it is only or chiefly by 

 comparison of these kinds with one another that gen- 

 eral laws are reached. Thus each of these departments 

 must be made co?nparative before it can become truly sci- 

 entific. Thus, then, the three fundamental departments 

 of zoology thus far found are (i) comparative anatomy, 

 (2) comparative physiology, and (3) comparative embryology. 



But the number and variety of animals is so great, 

 the material of science is so immense, that it is wholly 

 unmanageable and even bewildering unless arranged 

 and classified in an orderly way. Therefore animals 

 are divided and subdivided into groups, according to 

 their affinities or their differences and resemblances ; 

 those of the larger groups united with one another by 

 the most general characters, and separated from other 

 groups by the most profound differences, the smaller 

 groups being united by smaller but more numerous re- 

 semblances, and separated by less profound differences. 

 This is called (4) taxonomy, or classification. 



But such orderly arrangement can not be made with- 

 out extensive knowledge of animals in all parts of the 

 world, such as no one man can individually acquire. 

 Therefore it is necessary to describe all the kinds of ani- 

 mals, each in its proper place in the orderly arrange- 

 ment. But this can be done only by the co-operation of 

 all zoologists in all parts of the world, and the publica- 

 tion of these results, so that each can use the work of 



