THE LIVING ORGANISM 33 



sesses certain definite characteristics which give it a 

 particular place when animals more or less like it are 

 grouped or classified according to their degrees of re- 

 semblance and difference, in small genera of very similar 

 forms, in larger tribes or orders of similar genera, and in 

 more and more inclusive groups of these lesser divisions, 

 such as the classes and phyla, or main branches of the 

 animal tree. The conamon cat and its relatives are 

 even earlier to be regarded as anatomical subjects, and 

 their thorough analysis belongs to comparative anat- 

 omy, a name which explains itself. The purpose 

 of this department of natural history is to explore the 

 entire range of animal forms and animal structures, and 

 to determine the degree of resemblance and difference 

 exhibited by the general characters of entire organisms 

 and by the special qualities of their several systems 

 of organs. It provides the data from which classifica- 

 tion selects those which indicate mutual affinities with 

 greatest precision and surety. But its materials are 

 all the facts of animal structure, and because each and 

 every known organism can be and must be studied, 

 the investigator engaged in formulating the evidence 

 of evolution has at his disposal all the data referring to 

 the entire realm of animals. The data of embryology 

 are likewise coextensive with the territory of the animal 

 world, for we do not know of any form which does not 

 change in the course of its life history. An adult cat 

 is the product of a kitten which is itself the result of a 

 long series of changes from earlier and simpler conditions. 

 In so far as it deals with structures in the making, em- 

 bryology is a study of anatomy, but as it is concerned 

 primarily with all of the plastic remodeling which an- 



