The Making of Species 
plumes growing from the back, it cannot be said 
that the short tail of the vast majority of the 
waders is due to the fact that these birds would 
be at a disadvantage were their caudal feathers 
long. 
IsoLATION 
Isolation is a most important factor in the 
making of species. It is a factor to which 
Darwin failed to attach sufficient importance, 
and one which has been to a large extent 
neglected by Wallaceians. 
We have seen how a species can be improved 
or changed by natural selection. All those in- 
dividuals which have varied in a favourable 
direction have been preserved, and allowed to 
leave behind them offspring that inherit their 
peculiarities, while those which have not so 
varied have perished without leaving behind 
any descendants. Thus the nature of the species 
has changed. The old type has given place to 
a new one. Instead of species A, species B 
exists. This is what Romanes has called mono- 
typic evolution—the transformation of one species 
into another species. But any theory of the 
origin of species must be able to answer the 
question, Why have species multiplied? How is 
it that species A has given rise to species B, C, 
and D, or, while itself continuing to exist, has 
thrown off sister species B and C? How is it 
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