The idea of protective coloration is that very many animals have ordinarily come to be colored like the background on which they live. The process has taken many generations, and is very slow, but is none the less sure in the end. In most cases the animal is probably entirely unconscious of this point in its favor, and usually it does nothing to assist the deception. The result is none the less effective because the animals themselves are unconscious of the process. The cabbage worm is green in color like the cabbage. This does not mean that it got green by eating cabbage or by longing for greennesses. Through long years the enemies of the cabbage worm have been picking it off the plants on which it fed. This does not imply that cabbages as we know them are very old, but cabbage worms doubtless ate the leaves of the sea-kale long before man had cultivated it into cabbage. During all these years the enemies of the caterpillars, generally in the shape of birds, have been assiduously gathering them up. When we see how much the various members of the same human family may differ in complexion, how much the various pigs in the same litter may differ in size and in coloration, it is easy to understand that among these caterpillars which have eaten the cabbage there must have been considerable color variations. I do not imagine for a moment that the birds had any preference for any particular color in their cabbage worms. They took every caterpillar they saw, but they naturally first saw those that were least like the background on which they lived. The only caterpillar which was effectively hidden from his enemy was the one that was indistinguishable on the leaf. If it escaped in this way, the probabilities are that it would produce young which would be at least a little more likely to be green in color than the progeny of its darker-colored brothers and sisters. By this continued process the birds steadily weed out the darker-colored specimens. There would result, in the course of time, a race of caterpillars, whose ancestors for so many generations back had been light green in color, that there is little likelihood of any of the older and darker forms turning up again. In the course of time all dark tendencies will have disappeared from the family and practically all of the group will be light green. Any sport or variation in the shape of greater conspicuousness would fall a quick prey to the enemy and its line be cut off forever.