MIGRATION. 797 



indefinite impulse which led birds to scatter at the breeding season 

 and to hunt out safe hiding places for their nests; and that, as ene- 

 mies 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 improve- 

 ment 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 compli- 

 cated, 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 inter- 

 est 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 usu- 

 ally identical with that of the constituent individuals, it is not obvi- 

 ous 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 ani- 



