in nature--at least as continuing without modification for many
successive seasons), then accidental variations, if favourable,
would indeed accumulate and result in modification, without the aid
of the transmission of functionally produced modification. This is
true; it is also true, however, that only a very small number of
species in comparison with those we see around us could thus arise,
and that we should never have got plants and animals as embodiments
of the two great fundamental principles on which it is alone
possible that life can be conducted, {107a} and species of plants
and animals as embodiments of the details involved in carrying out
these two main principles.

If the earliest organism could have only varied favourably in one
direction, the one possible favourable accidental variation would
have accumulated so long as the organism continued to exist at all,
inasmuch as this would be preserved whenever it happened to occur,
while every other would be lost in the struggle of competitive
forms; but even in the lowest forms of life there is more than one
condition in respect of which the organism must be supposed
sensitive, and there are as many directions in which variations may
be favourable as there are conditions of the environment that affect
the organism. We cannot conceive of a living form as having a power
of adaptation limited to one direction only; the elasticity which
admits of a not being "extreme to mark that which is done amiss" in
one direction will commonly admit of it in as many directions as
there are possible favourable modes of variation; the number of
these, as has been just said, depends upon the number of the
conditions of the environment that affect the organism, and these
last, though in the long run and over considerable intervals of time
tolerably constant, are over shorter intervals liable to frequent