THE EVOLUTION OF LIVING BEINGS. 97 



in which the different types are perpetuated apoga- 

 mously. 



This gives us the clue as to the difference in distinctness 

 of the units within the Linneon and between the different 

 Linneons themselves; the units within each Linneon form 

 an intercrossing community, while the Linneons them- 

 selves usually do not intercross. 



Why not? 



Because mostly, individuals belonging to different 

 Linneons, have either an aversion to mating or are pre- 

 vented from mating, by isolation either in space (occur- 

 rence in different regions) or in time (different time of 

 flowering or different time of rutting) or by mechanic 

 obstacles (differences in size of the male of the one and 

 the female of the other Linneon, non-fitting copula- 

 ting organs etc.) , all this accentuated by the fact that in 

 many cases, even if mating occurs, no progeny or a 

 sterile progeny only, results. 



The distinctness of the Linneons is consequently caused 

 by the obstacles against mating of the individuals belon- 

 ging to different Linneons, which obstacles niay be rela- 

 tive obstacles which can be overcome, such as aversion f. i. 

 or final obstacles which cannot be overcome : innate steri- 

 lity asf. i. between many Linneons of the genus Verbas- 

 cum . 



Consequently it is nature itself which groups the indi- 

 viduals to Linneons and Linneons are thus something 

 more than mere conceptions of the human mind; it are 

 natural intercrossing communities of differently constitu- 



