Page:Carroll Lane Fenton - A History of Evolution (1922).djvu/56

 Rh ing the numbers of the oysters, which decrease would in turn cut down the numbers of the starfish, and so on. Thus we see that the maximum abundance of an organism is arbitrarily set by the conditions under which that organism lives. It may attain the limit set for it, but beyond that it may go only temporarily. Then the surplus dies from starvation, crowding, animal and plant enemies, and a thousand other of the factors which constantly work in the constant warfare of nature, the never-ending "struggle for existence."

The third factor of Darwinian evolution, that of natural selection, is based upon the other two. Darwin supposed that the individuals of a species, or variety, exhibited variations for two reasons: because it was part of their very nature to do so, and because the conditions of their environment forced them. In the course of this constant change there would, of necessity, be some modifications that were of value to their possessors, while others would appear which were of more or less definite harm. In the course of the struggle for existence, those creatures which possessed helpful variations would naturally possess a certain advantage over those which lacked it or which exhibited variations which were of harmful nature. Thus in a cold, snowy climate, that animal which developed a white coat would be much safer from detection than his companions which might have fur of a dark hue, either in approaching his prey, or in escaping his pursuers. The ultimate outcome of this would be