INDIVIDUALS AND SPECIES 27 



taking the former. Whichever course may be 

 followed, the difficulties of systematic treatment 

 which have inspired most discussions remain the 

 same. Species differ in their nature. The same 

 criteria are frequently inadequate for their separa- 

 tion in two unrelated groups. Morphological and 

 physiological differences, fertility, mutability, all 

 play a part in taxonomy. Their relative values 

 must depend upon their effectiveness in special 

 cases, and the taxonomist's idea of species is likely 

 to be influenced by the useful characteristics of 

 the group in which he works. 



Attempts have been made from time to time to 

 clarify the term species by simplifying its applica- 

 tion. Elementary species, which are approximately 

 the same as the pure lines and pure-line equiva- 

 lents of biology, are possibly of some use in experi- 

 mental work, but they are more definitely limited 

 than the species of taxonomy only in degree. In 

 certain characters they are uniform. The recogni- 

 tion of elementary species does not, moreover, solve 

 the species problem. They are not independent 

 natural entities but only components of natural 

 species, which remain to be explained. For this 

 reason the term species is used in these pages to 

 designate natural species. 



The relationship of individual and species has 

 another aspect which can best be brought out by a 

 broader consideration of living units. 



