70 PROBLEMS OF EVOLUTION 



commonly found, the mature plumage is not assumed at any 

 rate till after the third year. Yet next spring you find that the 

 majority are birds that have doffed the brown of childhood and 

 donned the white and grey of mature life. What has become of 

 the one-year-olds and two-year-olds ? 



Sea-gulls can stand a great deal in the way of cold. No 

 severity of frost prevents them from fishing or even bathing 

 for pleasure. But it is difficult to imagine that this hardiness 

 can be maintained by any means but elimination. The difficulty 

 of finding food must dispose of a great many ; they often fish 

 and catch nothing. Storms may carry not a few out to sea. 

 Death in some form has claimed them. 



What an enormous host of migratory birds start southward 

 from our shores in autumn ! During their stay with us they 

 have certainly trebled their numbers and yet next spring there 

 are no more than the last. Most of the young birds have gone 

 astray, in stormy weather have failed to reach the shore, or have 

 failed to get food or have met with enemies. If migration 

 entails such a death-rate, it has been urged, why have birds 

 formed so foolish a habit? But those species that winter at 

 home do no more than keep up their numbers and in that the 

 migrants also succeed. 



In the two cases mentioned physical conditions act very 

 severely and it is not probable that in England birds of prey 

 and other enemies play more than a secondary part. But we 

 may be sure that it was otherwise during past ages when birds 

 we are familiar with were being evolved. Even now there are 

 sparrow-hawks, crows, magpies and jays in plenty in our islands 

 and they are constantly at work on small birds or their eggs. 

 Birds of prey able to tackle a sea-gull are not so common, but 

 they are by no means extinct. And in addition to these beasts 

 of prey there is the rapacious and almost ubiquitous boy birds- 

 nester and in the case of rare species, the pestilent adult 

 collector. 



With most animals, now as in the past, the struggle is 

 mainly one against enemies. Antelopes are pursued by lions 

 and other beasts of prey. The tiger is the greatest danger 



