DOCTRINE OF EVOLUTION AND RELATED IDEAS 495 



495. Isolation. When new variations occur it is clear that 

 they will have a better chance to accumulate and result in a 

 new and permanent stock if for any reason these new characters 

 cannot be "swamped" or buried by crossing with the more 

 numerous and conservative members of the parent stock. 



The effects of geographic or physical isolation is seen on is- 

 lands which have been populated from the mainlands, or in 

 neighboring lakes, or in valleys between which there are barrier 

 mountains. In such conditions the animals and plants are dis- 

 tinctly different from the species from which they are believed 

 to have sprung, and those on different islands are different from 

 one another. It is believed by many observers that a larger 

 number of related species are found under such conditions than 

 where there is free interbreeding and migration. 



It is possible also that there are internal hindrances to mating 

 which would operate in much the same way as geographic iso- 

 lation. There may arise some instinctive feeling which would 

 prevent mating between the average members of a species and 

 individuals that had varied considerably from that average. 

 We see this in some degree in human races. Or changes in the 

 copulating organs or in the reactions of the germ cells might 

 render such free intercrossing impossible. 



Finally, we have seen in Mendel's experiments that certain 

 unit characters, even when the sperm and ova unite freely, do 

 not permanently coalesce and blend; but sooner or later may 

 separate out pure again. In a sense this is a case of physiological 

 isolation of the structures that carry the varying characters 

 even in the union of germ cells. 



We may say then that the varying animals may be kept 

 apart by geographic barriers; or the germ cells may be kept 

 apart even when the animals live freely together; or the char- 

 acter-bearing determiners may be kept apart even when the 

 germ cells come together. 



496. The Environment. Assuming that variations occur 

 and make evolution possible, and that heredity enables some of 

 these variations to pass from one generation to another and thus 

 to accumulate, and that isolation keeps the various individuals 



