EVOLUTION AS A NATURAL PROCESS 119 



their complexity does not lessen their validity and 

 efficiency as the immediate factors of the process. 



Taking up the first proposition, we return to the 

 subject of variation that has been discussed previously 

 for the purpose of demonstrating its reality. The 

 observations of every day are enough to convince us 

 that no two living things are ever exactly alike in all 

 respects. The reason is that the many details of organic 

 structure are themselves variable, so that an entire 

 organism cannot be similar to another either in material 

 or in functional regards, while furthermore it would 

 be impossible for an animal to be related to environ- 

 mental circumstances in the same way as another 

 member of its species unless it was possible for two 

 things to occupy the same space at the same time ! 

 Individual differences in physical constitution are 

 displayed by any litter of kittens, with identical parents ; 

 it needs only a careful examination to find the varia- 

 tions in the shape of the heads, the length of their 

 tails, and in every other character. Sometimes the 

 differences are less evident in physical qualities than 

 in disposition and mental make-up, for such variations 

 can be found among related kittens just as surely as 

 among the children belonging to a single human family. 



Not only do all organisms varx-jiut they^-seem to 

 vary in somewhat similar ways. While modern investi- 

 gations have thrown much hght upon the relations be- 

 tween variations and their causes, of particular value 

 in the case of the congenital phenomena, the greatest 

 advance since Darwin's time consists in the demon- 



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