We touch here the crucial point of evolution in its relation to Darwin's principle of natural selection. Darwin says that he did not at first realize the overwhelming influence of the mass in its swamping effects on the individual variant. He made a very important concession to this view in the later editions of the "Origin of Species," and thought it necessary to assume that for a new form to arise it must first appear in a large number of individuals.
But to-day the situation has changed and new facts have come to light—facts that remove the enormous difficulty that Darwin met by what may seem now to have been an unnecessary concession.
An imaginary case will illustrate what I wish to say. Suppose that a species consisted in each generation of a million individuals and let us imagine that a new character—a definite variation—appears in an individual. The individual that bears it will pair with another ordinary individual and transmit its new character to all of its offspring. In order to simplify our case let us imagine that from each pair of individuals four reach maturity. The million of individuals has increased to two millions, but accidents and competition may kill off one million of these, so that the race is again reduced to its standard of one million. If, then, we suppose that two of the new kinds of individuals survive on the average, and pair at random, there will be eight in the next generation (in reality only six of the eight will show this character). If these survive they will transmit their character to twelve of their offspring. Gradually, however, step by step, the new character will be added to the whole race. Thus any new, definite character will gradually appear in all the individuals whether it is useful or not. If it is useful it may sooner implant itself on the race than if it is indifferent; for more individuals may survive that possess it, than of those without it. It will spread faster, but in any case it will come in the long run. Thus we see that it spreads, not because it is advantageous, but because it is a definite variation. Injurious characters will have greater difficulties in inflicting themselves on the race, and if distinctly injurious may never succeed.
While one character is spreading, other definite variations may also be adding themselves to the race. Those individuals that combine the greatest number of useful additions will have the best chance of survival. Slowly the race advances in the direction of the sum of its advantages and adaptation; success, not in one but in several characters, is the true criterion of survival.
To fix our attention on each single advantage and to ascribe to it alone the palm of victory gives an incomplete idea of the progress of evolution, for evolution follows the line of the greatest number of adaptations. Success in every generation cannot be traced to one variation, but to the sum of all mingled advantages.