MIGRATION. 793 



of temperature, permits birds to divide their time between widely 

 separated regions, and, whether the choice be conscious or uncon- 

 scious, the breeding places of migratory birds are selected on account 

 of their safety, and not because they furnish all that a permanent 

 home must supply. If we believe, with Professor Marsh, that the 

 power of flight was acquired by birds after they became arboreal, we 

 must look for the primitive home of the migratory birds in the great 

 tropical and subtropical forests where arboreal reptiles and arboreal 

 mammals still abound; nor can we believe the great armies of north- 

 ern birds which find abundant food in southern lands in winter are 

 driven out by scarcity on the approach of spring. Enemies are 

 numerous in the tropics, but no animals have sharper senses or better 

 means of escape than birds, and, trusting in their power of flight and 

 their quick sight and hearing, they venture into danger with con- 

 fidence. The great charm of birds to us is the fearlessness with 

 which they approach man, who is the most dreaded enemy of all 

 other vertebrates; but while adult birds are eminently fitted for 

 taking care of themselves, the opposite is true, in even greater de- 

 gree, of nestlings, for no animals are at the same time more helpless 

 and more exposed to danger than many young birds, while eggs are 

 not only absolutely helpless but also very tempting to enemies, 

 although there is no group of animals in which the safety of the eggs 

 and young is more important. Among birds a high birth rate is in- 

 compatible with flight, 'for their eggs are large and heavy, and the 

 preservation of each species imperatively demands that every egg 

 shall be cared for with unceasing solicitude; for while, in other 

 animals, increased danger to eggs and young may be met by an 

 increase in the birth rate, this can not be much increased in birds 

 without corresponding loss in the power of flight. Every one knows 

 how quickly birds are exterminated by the destruction of their eggs 

 and young, and the low birth rate of all birds of powerful flight is a 

 sufficient reason for migration, for at the same time that flight 

 limits the birth rate it permits the birds to seek nesting places 

 beyond the reach of their enemies; and as there is rigorous selec- 

 tion of the nestlings which are born in safe nests, it is easy to 

 understand how the instinct has been gradually fixed by selection, 

 and how, as it has become more and more firmly fixed, and as the 

 safety of the eggs and young has become assured by the remoteness 

 and isolation of the nests, the birth rate has been still more reduced 

 and the power of flight correspondingly extended. Many sea birds 

 that nest on desolate rocks in midocean lay only one egg each year, 

 and have the power of flight in its highest perfection. The power 

 of the storm petrel to wander is as boundless as the ocean, and 

 while it lays only a single egg, it is said to be the most prolific 



