' I observe, also, that in the very passage
in which he does so, Mr. Lewes appears to have been misled by Mr.
Darwin's use of that dangerous word "means," and, at the same time, by
his frequent treatment of natural selection as though it were an active
cause; so that Mr. Lewes supposes Mr. Darwin to have fallen into the
very error of which, as I have above shown, he is evidently struggling
to keep clear--namely, that of maintaining natural selection to be a
"cause" of variation. Mr. Lewes then continues:--

"He [Mr. Darwin] separates Natural Selection from all the primary causes
of variation either internal or external--either as results of the laws
of growth, of the correlations of variation, of use and disuse, &c., and
limits it to the slow accumulation of such variations as are profitable
in the struggle with competitors. And for his purpose this separation is
necessary. But biological philosophy must, I think, regard the
distinction as artificial, _referring only to one of the great factors
in the production of species_."[355]

The fact that one in a brood or litter is born fitter for the conditions
of its existence than its brothers and sisters, and, again, the causes
that have led to this one's having been born fitter--which last is what
the older evolutionists justly dwelt upon as the most interesting
consideration in connection with the whole subject--are more noteworthy
factors of modification than the factor that an animal, if born fitter
for its conditions, will commonly survive longer in the struggle for
existence. If the first of these can be explained in such a manner as to
be accepted as true, or highly probable, we have a substantial gain to
our knowledge. The second is little--if at all--better than a truism.
Granted, if it were not generally the case that those forms are most
likely to survive which are best fitted for the conditions of their
existence, no adaptation of form to conditions of existence could ever
have come about. "The survival of the fittest" therefore, or, perhaps
better, "the fertility of the fittest," is thus a _sine qua non_ for
modification. But, as we have just insisted, this does not render "the
fertility of the fittest" an especial "means of modification," rather
than any other _sine qua non_ for modification.

But, to look at the matter in another light. Mr. Darwin maintains
natural selection to be "the most important but not the exclusive means
of modification."

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