The Making of Species 



fact that his colour is a matter of comparative 

 indifference to the species; in consequence plenty 

 of latitude is allowed to his tendency to vary. 



Our view, then, is that evolution proceeds by 

 mutations, which may be large or small. 



The mutation is the result of a rearrangement 

 in part or parts of the fertilised egg, and this re- 

 arrangement shows itself in the adult organism as 

 a change in one or more of its characteristics. 

 The mutation may be correlated with only one 

 of the sexual organs, and when this is the 

 case, it gives rise to the phenomenon of sexual 

 dimorphism. The appearance in the adult of 

 certain, if not of all, characteristics is affected by 

 causes other than the nature of the biological 

 molecules from which they are derived. The 

 tendency to develop in a certain direction is 

 there, but something else, such as the secretion 

 of hormones from the sexual cells, is frequently 

 necessary to enable a given tendency to fully 

 develop itself. Thus it is that castration often 

 affects the bodily appearance of those animals 

 operated on. When a mutation appears, natural 

 selection decides whether or not it shall persist. 



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