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. 
344 
