CHAPTER II 
EVOLUTION 
EVOLUTION may be defined as progress involving dif- 
ferentiation, an ever-growing complication of things 
which accompanies almost all the operations of Nature. 
The idea of a differentiation of this kind may be en- 
forced by a homely and quite imaginary illustration 
of such a process. Imagine the proper ingredients of 
a plum cake to be very finely minced and intimately 
mixed together, so as to form a more or less homo- 
geneous material. Then, if by any means the separate 
particles of currants, raisins, peel, and so forth, could 
be made to segregate out in such a way as to give rise 
to the ordinary structure of this pleasant confection, 
we should have arrived at the structure of a plum cake 
by a process of evolution involving considerable dif- 
ferentiation. 
The progressive increase in complexity which is 
characteristic of so many natural processes is in great 
part occasioned by the fact that a single ‘cause’ is 
followed as a general rule by more than one ‘ effect.’ 
This apparently simple circumstance was pointed out 
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