62 



GENETICS 



fering from species perhaps only by the addition 

 or subtraction of a single character, and thus the 



Fig. 31. — Diagram to illustrate various ideas about "species." Under 

 Species A are represented two groups of individuals which are near 

 enough alike to be placed within a single species, but which are suffi- 

 ciently unlike each other to constitute the " sub-species " or "varie- 

 ties " of Darwin. Under Species B are various groups of individuals 

 distinguished from each other by the addition or loss of one or more 

 characters. These groups represent the "elementary species" and 

 "varieties" of de Vries. 'The "barrier of Linnaeus" attempted to 

 separate species absolutely from each other. Darwin sought to find 

 loopholes in this barrier. To-day attention is directed rather to the 

 relation between individuals than to the boundaries between species. 



possibilities of analytical classification have become 

 almost limitless. 



An elementary species, according to de Vries, is a 

 progressive mutation differing from the type species 



