74 ISOLATION AS A FACTOR 



found among plants in which cross-fertilization 

 is not general, among Protozoa and other low 

 forms in which specific distinctions are unknown 

 or at least obscurely shown, in cases of isolation 

 other than geographical, and in a few cases which 

 seem to be explainable on the ground of re- 

 invasion. It is possible that species once thor- 

 oughly separated through some form of geo- 

 graphical segregation may later invade the terri- 

 tory, the one of the other, without crossing or 

 hybridization. This seems likely to occur among 

 plants, and it is possible among migratory ani- 

 mals also. Taking the world over, re-invasion is 

 probably not a rare phenomenon, although in 

 most cases the invading species may fail to estab- 

 lish itself. In the case of animals dependent on 

 man, we find sometimes a form of political segre- 

 gation, which may lead to the separation of races 

 without actual physical barriers. The races of 

 sheep in England, for example, go by counties. 

 The artificial boundary of a county is a barrier 

 to man, rather than to the sheep. In all forms 

 of artificial selection, a corresponding degree of 

 artificial segregation is always implied and, with- 

 out segregation, selection has no eflFectiveness in 

 race-forming. Nothing, for example, can be 

 done for the race improvement of fishes, unless 

 these can be segregated in artificial ponds, away 

 from the unselected mass of the species. 



