52 CONTROLLED NATURAL SELECTION 
in this way, there will be evolution. Highly 
evolved species will be found in highly com- 
plex and often restricted environments ; such 
species will be found to vary very little. 
High specialisation demands great fixity of 
structure. The local addition of a factor 
will produce local evolution and the estab- 
lishment of a sub-species. 
Reasons have been given why one cannot 
conceive of a species being displaced from its 
specific environment by the variety of another : 
for similar reasons, one cannot conceive of a 
variation of a species being able either to 
occupy an environment rendered vacant by 
the extinction of a species, or to discover an 
entirely new environment. Evolution equals 
environmental differentiation, the better ap- 
preciation by organic matter of the diversities 
of its environment. The question of com- 
petition yet remains to be considered. As 
has already been shown, this can only occur 
within a common environment, and then 
only when the common environment is not 
sufficient to accommodate the two or more 
species which share it. Suppose (see Fig. 6) 
S, and S, are two species, E, and E, 
are their specific environments, and E, a 
common environment, where competition is 
