onment of the young {139}
plant, and it is in this latter capacity that it affects its offspring.
Wherever, as in plants and mammals, the organism is parasitic upon the
mother during its earlier stages, the state of nutrition of the latter will
almost certainly react upon it, and in this way a semblance of transmitted
weakness or vigour is brought about. Such a connection between mother and
offspring is purely one of environment, and it cannot be too strongly
emphasised that it has nothing to do with the ordinary process of heredity.

The distinction between these two kinds of variation, so entirely different
in their causation, renders it possible to obtain a clearer view of the
process of evolution than that recently prevalent. As Darwin long ago
realised, any theory of evolution must be based upon the facts of heredity
and variation. Evolution only comes about through the survival of certain
variations and the elimination of others. But to be of any moment in
evolutionary change a variation must be inherited. And to be inherited it
must be represented in the gametes. This, as we have seen, is the case for
those variations which we have termed mutations. For the inheritance of
fluctuations, on the other hand, of the variations which result from the
direct action of the environment upon the individual, there is no
indisputable evidence. Consequently we have no reason for regarding them as
playing any part in the production of that succession of temporarily stable
forms which we term evolution. In {140} the light of our present knowledge
we must regard the mutation as the basis of evolution--as the material upon
which natural selection works. For it is the only form of variation of
whose heredity we have any certain knowledge.

It is evident that this view of the process of evolution is in some
respects at variance with that generally held during the past half century.
There we were given the conception of an abstract type representing the
species, and from it most of the individuals diverged in various
directions, though, generally speaking, only to a very small extent. It was
assumed that any variation, however small, might have a selection value,
that is to say, could be transmitted to the offspring. Some of these would
possess it in a less and some in a greater degree than the parent. If the
variation were a useful one, those possessing to a rather greater extent
would be favoured through the action of natural select