THE EVOLUTION OF ANIMALS. 393 



to produce permanent and separate new stock, it is at 

 least advantageous that the newly varied animals be 

 isolated from the conservative, less variable relatives. 

 This may come about through geographical isolation. A 

 barrier, such as a mountain range or an arm of the sea, 

 may come to lie between two kindred stocks in such a way 

 that their special variations and adjustments may be 

 kept pure, and thus new species are made possible through 

 the isolation of varieties. This has doubtless been of the 

 greatest importance in the history of animals on the 

 earth. 



It is also known that a similar result, in keeping strains 

 separate, is sometimes obtained even when some kinds 

 of variants are crossed freely with their parent stock. A 

 certain definite per cent, of the ultimate offspring, in such 

 crosses, will show unmixed the particular characteristics 

 of each of the parents. That is to say, a part of the off- 

 spring will be pure, like one parent; and another part 

 will be pure, but like the other parent. It appears as 

 though the qualities of the original parents are kept apart 

 and pure (isolated) in the union of the germ cells them- 

 selves, without geographical isolation, and are trans- 

 mitted unchanged to the next generation. The student 

 will find this subject discussed in the reference books 

 under "Mendel's laws" of heredity. 



392. The Environment. Assuming that variations oc- 

 cur and make evolution possible, and that heredity en- 

 ables these variations to pass from one generation to 

 another and thus to accumulate, and that isolation keeps 

 the various individuals from evening up and thus destroy- 

 ing their variations by inter-crossing, it still remains to 

 find the factors that have determined and guided the 



