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 their freelycrossingwith 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 



