112 THE FOUNDATION'S OF ZOOLOGY 



Most animals owe their existence to the occurrence, in their 

 natural home, of all that their life requires, but the power to 

 traverse great distances at great speed, and to pass over all the 

 barriers of land and water, joined to their indifference to changes 

 of temperature, permits birds to divide their time between two 

 widely separated regions; and whether the choice be conscious or 

 unconscious, the breeding places of migratory birds are selected 

 on account of their safety and not because they furnish all that 

 is needed for a permanent home. 



If we believe, with Professor Marsh, that the power of flight 

 was acquired by birds after they became arboreal, we must look 

 for the ancestral home of the migratory birds in the great tropi- 

 cal and sub-tropical forests, where arboreal reptiles and arboreal 

 mammals still abound, nor can we believe the great armies of 

 northern 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 are more 

 alert, or have sharper senses, or better means of escape, than 

 birds, and, trusting to their powers of flight, and their quick sight 

 and hearing, they venture into danger with confidence, for the 

 great charm of birds to us is the fearlessness with which they 

 approach man, who is the most dreaded enemy of all other verte- 

 brates. But while this is eminently true of adult birds, its opposite 

 is true, in even greater degree, of nestlings ; for no animals are 

 at the same time more helpless and more exposed to danger than 

 many young birds, while the exposed eggs are of course abso- 

 lutely helpless, and very tempting and attractive to enemies, 

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

 eggs and young is more important. As their eggs are very 

 large and heavy, a high birth-rate is incompatible with flight, 

 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 or young may 

 be met and compensated by an increase in the birth-rate, the 

 birth-rate of birds cannot be much increased without a corre- 

 sponding restriction of the power of flight. Every one knows 

 how quickly birds may be exterminated by the destruction of 

 their eggs or young, and the low birth-rate of all birds of power- 



