376
In type-producing selection, the circumstances which sur
round all the animals experimented upon are made as
uniform as possible ; the variations from which selection
is made are, for the most part, those variations which are
inevitably associated with sexual reproduction ; and similar
variants are prevented from interbreeding with individuals
unlike themselves by a physical barrier. This process is
continued for several generations until the object which
the fancier has set before himself has been attained. Now
let us see how far it is possible that this method should
obtain in nature. It seems to be probable, if we allow
the tacit assumptions which the doctrine of Natural Selec
tion takes for granted. If variations can occur in nature
which are solely the result of sexual reproduction, if ex
ternal conditions produce no direct effect upon organisms,
if necessary birth variations are the sole material for selec
tion to act on, then it might appear probable that species
should be transmuted, whether the necessary isolation for
breeding purposes be effected by an impassable barrier
or by the survival of the similar favourable variations and
the destruction of the rest. But in order that species
should be transmuted in nature by physical separation,
we must make further assumptions. Similar variations
must first arise in at least a few individuals of a given
group ; these must be isolated by a physical barrier, so
that they breed only with one another. Moreover, in the
next generation similar favourable variations must again
arise, apart from the direct or indirect effect of circum
stances, and these in their turn must be separated.
But is it likely that this will take place? It is difficult
to understand how such similar variants should be
collected together and then isolated by river or moun
tain, or some natural barrier, in each generation, unless
indeed they separated themselves, and then no bar