FOR DARWIN 377 



"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 conces- 

 sion 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. y 



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 indi- 

 viduals 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 gen- 

 eration (in reality only six of the eight will show this character). If 

 these survive they will transmit their character to twelve of their off- 

 spring. 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 infiicting 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 varia- 

 tion, but to the sum of all mingled advantages. 



