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 selection at the expense of their less fortunate brethren, and would leave a greater number of offspring, of whom some possessed it in an even more marked degree than themselves. And so it would go on. The process was a cumulative one. The slightest variation in a favourable direction gave natural selection a starting-point to work on. Through the continued action of natural selection on each successive generation the useful variation was gradually worked up, until at last it reached the magnitude of a specific {141} distinction. Were it possible in such a case to have all the forms before us, they would present the appearance of a long series imperceptibly grading from one extreme to the other.