266 Darwin, and after Darwin. 
Tennyson thus noted the fact, and a few years later 
Darwin supplied the explanation. 
But of course in many, if not in the majority of 
cases, anything that adds to the life-sustaining power 
of the single life thereby ministers also to the life- 
sustaining power of the type; and thus we can under- 
stand why all mechanisms and instincts which minister 
to the single life have been developed—namely, 
because the life of the species is made up of the lives 
of all its constituent individuals. It is only where 
the interests of the one clash with those of the other 
that natural selection works against the individual. 
So long as the interests are coincident, it works in 
favour of both. 
Natural sclection, then, is a theory which seeks to 
explain by natural causes the occurrence of every kind 
of adaptation which is to be met with in organic 
nature, on the assumption that adaptations of every 
kind have primary reference to the preservation of 
species, and therefore also, as a general rule, to the 
preservation of their constituent individuals. And 
from this it follows that where it is for the benefit of a 
species to change its type, natural selection will effect 
that change, thus leading to a specific transmutation, 
or the evolution of a new species. In such cases 
the old species may or may not become extinct. If 
the transmutation affects the species as a whole, or 
throughout its entire range, of course ¢ha¢ particular 
type becomes extinct, although it does so by becoming 
changed into a still more suitable type in the course 
of successive generations. If, on the other hand, 
the transmutation affects only a part of the original 
species, or not throughout its entire range, then the 
