DOCTRINE OP EVOLUTION AND RELATED IDEAS 505 



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 (genes) 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 

 from evening up and thus destroying their variations by inter- 

 crossing, it still remains to find the factors that have determined 

 and guided the actual course of evolution in any given instance. 

 Why has development taken the course it has ? 



Theoretically, as we have seen, there might be an inherent 

 tendency in living matter at the beginning to vary or evolve in 

 a certain direction. Biologists are not agreed upon the exist- 

 ence of such a tendency. On the other hand, the environ- 

 ment — meaning the total external conditions of the hfe of 

 organisms — may guide or direct the course of development. 

 Any guidance must come from the one or the other of these 

 sources, — internal or external. 



The environment may act in either of two ways to mold 

 evolution. We have seen in the first place that the environment 

 does act directly to produce specific changes in organisms. 

 If these changes can be inherited, this will be a most important 

 means of guiding evolution. It is not certain, however, that 



