39 2 ELEMENTARY ZOOLOGY. 



vary. All organisms vary. Variation suggests two or 

 three things : (i) An animal may differ from day to day as 

 the result either of its own activities or of the action of the 

 environment upon it; (2) it may differ, at any stage or 

 condition of its life, from what its parents were at the 

 corresponding stage; and (3) it may differ also from its 

 brothers and sisters even at the same stage of life and 

 under similar conditions. Some of these changes are 

 evidently caused by conditions outside the organism; 

 others by internal conditions. These changes make 

 evolution possible. Whether all thr asses of changes 

 enter equally into evolution we cannot say. One of the 

 most important questions to the modern biologist is 

 this: "What produces the variation actually found in 

 individuals of a species, and to what extent are these 

 variations due to internal or to external causes?" 



390. Heredity. While variation is necessary and pro- 

 duces some evolution of the individual, it does not insure 

 racial evolution. Evolution of the race cannot take place 

 unless, in some way, the individual variations are trans- 

 mitted and accumulated. Heredity, the power to trans- 

 mit new variations as well as the older characteristics, 

 is therefore an agent in evolution. It does not follow 

 that all new characters that an animal gets are subject 

 to transmission; but without some means of passing 

 variations from parent to offspring no evolution is 

 possible. 



391. Isolation. If new variations occur and these 

 variable forms are left to cross freely with relatives that 

 have not varied, there is a tendency for the new quality 

 to be "swamped" or buried. In order that variations 

 that can be transmitted may be kept pure and in a position 



