98 THE EVOLUTION OF LIVING BEINGS. 



ted types. As we have seen that constant intercrossing, 

 such as takes place inside of many Linneons between 

 the different types, by the selection of a certain type, 

 which is the rule in nature, has finally a swamping effect, 

 leading to an overwhelming majority of the dominant 

 type including both pure dominants and hybrids 

 indistinctible from these at sight there is no reason 

 to suppose that what happens within the Linneon 

 would not happen between the several Linneons them- 

 selves, if they also intercrossed freely, so that it is reaso- 

 nable to suppose that if there were no obstacles of any 

 kind to a free intercrossing in nature between all the 

 differently constituted types, which people the earth, 

 this latter would be chiefly peopled by one type only. 



The cause of the possibility of a great diversity of 

 types, living side by side in the same regions, is the exis- 

 tence of obstacles to free intercrossing, isolating these 

 apparently non-isolated types as effectively as if the 

 different types were put into separate cages. 



Within each cage intercrossing freely occurs and 

 leads, by the aid of selection, to a high degree of pheno- 

 typical uniformity, while the numerous phenotypically 

 different types so obtained the Linneons remain 

 distinct because the walls of the cages in nature the 

 obstacles to crossing keep them separate. 



Linneons consequently, though being themselves the 

 vestiges of the result of a cross, are kept distinct in nature 

 by obstacles against theirfreelycrossingwith other Linneons. 



If there existed no obstacles to unlimited intercros- 

 sing in nature, we would be unable to distinguish Lin- 

 neons ; if no crossing took place at all, Linneons being 



