WHAT IS A SPECIES? l6l 



activity of the whole as constructed. Thus the likeness in 

 form and function, which leads to the classification of organ- 

 isms as of the same species, does not arise by virtue of like- 

 ness of the matter involved in its construction, but by virtue 

 of likeness of the agency by which the particular construction 

 is brought about. To put this proposition in concrete form, a 

 particular cat has the form and function it possesses, not by 

 virtue of any qualities inherent in the bones or muscles or tis- 

 sues of which it is composed, or in the cells or in the ultimate 

 chemical elements of which it is composed, but its individual 

 characteristics are altogether determined by the fact that it 

 developed from a cat which was its mother. 



Descent is the explanation of the particular characteristics 

 of each individual. In dealing with such characteristics, we 

 are dealing with the phenomena of life which are continuous, so 

 far as our experience tells, and depend for their expression 

 not alone upon the immediate surroundings of the individuals, 

 but upon pre-existing living organic individuals, its ancestors. 



