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this selection is found in the struggle for existence arising 
from the tendency of all organisms to increase in a 
geometrical ratio. 
But when this assertion is made, it is tacitly assumed 
that sexual reproduction will necessarily cause the output 
of variations which are unmodified by any other influence ; 
that these variations will afford the necessary material for 
selection ; and that Natural Selection—selection by life 
and death—is the only isolation for breeding purposes 
possible in nature. But if the first two assumptions are 
taken for granted, it does not necessarily follow that 
Natural Selection is the sole agent in nature for isolating 
similar variations. And, first, it may be observed that, 
on the principles of Natural Selection, isolation by life 
and death cannot be the only possible means of isolating 
similar variants for breeding purposes; for in artificial 
selection this, is generally effected by physical separation ; 
and hence we may presume that there may be two or 
more methods of isolation in nature. The next point to 
be observed is that it is the fact of isolation which is 
important, and that the same result may be attained by 
many different methods. As Mr. Romanes says :— 
“All that the causation of the case requires is that there should be 
exclusive breeding between the similarly modified individuals. Whether 
this exclusive breeding is secured by killing off all other individuals, 
or by fencing them all out of a field, is plainly immaterial.”—(Con 
temporary Review, vol. liii.,p. 846.) 
The same remarks will apply with equal force to social 
segregation and to physiological differentiation. 
This is true as to the immediate result of the isolation 
of similar variants, but it is not immaterial what methods 
are suggested ; there may be methods practicable to art 
which would be impossible in nature ; methods appearing 
like a splendid vision in the realm of theory and yet not