Page:Popular Science Monthly Volume 52.djvu/819

Rh indefinite impulse which led birds to scatter at the breeding season and to hunt out safe hiding places for their nests; and that, as enemies also improved in power to find the most accessible nests, the instinct has been gradually shaped into definiteness by extermination and natural selection, until at last safe breeding grounds, far away from home and far out of the reach of natural enemies, have become established, and until many species and all the members of each species have come to share the impulse to resort to the selected breeding places on the approach of sexual excitement, and to follow the same path between distant points; that the increasing safety of the eggs and young has permitted a low birth rate and the improvement by selection of the power of rapid and long-continued flight; and that this has in its turn permitted the migration to become longer and longer, and more and more protection to the eggs and young.

The history of migratory birds has been long and complicated, and there has been time for great changes in climate and in the distribution of land and water, and these have no doubt left some permanent impression on the habits of birds. The birds have not eluded all their enemies, for predaceous birds and their prey are found together at both ends of the journey. New ways to escape enemies and new ways to find food are as important as they ever were, and the details of the subject are very complicated, although it seems clear that its broader outlines admit of explanation without recourse to geological changes or the inheritance of the direct effects of the conditions of life.

In conclusion, I wish to remind the reader that our present interest in migration lies in its value and simplicity as an illustration of the general law that the adaptations of Nature are for the good of the species and not for the benefit of the individual.

This law is universal, but since the welfare of the species is usually identical with that of the constituent individuals, it is not obvious unless the good of the species demands the sacrifice of individuals.

Long journeys are hazardous. Every California salmon which enters on the long journey to the breeding ground is destroyed, and the whole race of adult fishes is wiped out of existence, for the good of generations yet unborn. Few shad ever return to the ocean, and storm and accident and ruthless enemies work their will on the migrating birds and decimate them without mercy, although the dangerous return to the safe breeding grounds still keeps up, in order that children which are yet unborn may survive to produce children in their turn.

The safeguards which Nature throws around eggs and infants and the immature, and the indifference to the fate of the mature