THE EVOLUTION OF LIVING BEINGS. 157 



A Linneon of the class here treated, once having be- 

 come monospecific, must sooner or later be extermina- 

 ted without leaving any descendant, unless it crosses 

 with a form belonging to another Linneon. Such a pro- 

 geny of course is not identical with the parent-spe- 

 cies, but gives rise to new species. 



The conclusion therefore is : species arise by crossing, 

 perpetuate themselves by heredity and are gradually ex- 

 terminated by the struggle for life, those last exterminated 

 obtaining the epitheton omans: selected ones. 



The result of a cross consequently can lead to the for- 

 mation of new species but need not do so ; it always 

 must lead to the production of new forms. 



If however such new forms continue to intercross, 

 no new species will ever be formed; usually however 

 crossing is not so promiscuous as all that, and new spe- 

 cies do arise by segregation ; to such applies what has 

 been said above. 



Among the other new forms, promiscuous crossing 

 sooner or later is limited also to intercrossing within 

 certain groups, between which bars against intercros- 

 sing are erected by isolation, aversion or sterility. 



Such smaller intercrossing communities are also uni- 

 ted by systematists to Linneons. 



They can resist untoward circumstances much bet- 

 ter than Linneons, consisting of strict self fertilizers, 

 because they have a far greater degree of plasticity 

 through the fact, that the intercrossing of the forms wi- 

 thin them, gives rise continually to the birth of new 

 forms, offering new material, resistant to the extermi- 

 nating effect of the struggle for life. 



u- 



