MIGRATION IN ITS BEARING ON LAMARCKISM II3 



ful flight is a sufficient reason for migration, for at the same 

 time that their fitness for flight limits the birth-rate, it permits 

 them to seek nesting places beyond the reach of their enemies ; 

 and as there is rigorous selection of the nestlings which are born 

 in safe nests, it is easy to understand how the instinct has been 

 gradually acquired by selection, and how, as it has become more 

 and more flrmly 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 still more extended. Many sea-birds, which make their 

 nests on desolate rocks in mid-ocean, lay only a single egg each 

 year and exhibit 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 ^Zg, there is reason to 

 believe that it is the most prolific of all birds, for the number of 

 individuals is said to be greater than in any other genus. 



We cannot believe that all migratory birds inherit the habit 

 from some common parent which was migratory, nor is it proba- 

 ble that, in all cases, it owes its origin to the same influences; 

 but if the view here advanced be correct, we must believe that, 

 in most migratory birds, it has been brought about by the needs 

 which arise in connection with reproduction, and not by the sup- 

 ply of food, and that the winter home in tropical and sub- 

 tropical regions, and not the birthplace of modern birds, must 

 be regarded as the original starting-point for the migratory habit. 



While Wallace was the first to recognize the importance of 

 selection in the formation of this as well as other habits and 

 instincts, he seems to regard selection alone, without the assist- 

 ance of geological changes, as inadequate to explain all the facts 

 of migration. He says: "It appears to me probable that here, 

 as in so many other cases, survival of the fittest will be found 

 to have had a powerful influence. Let us suppose that in any 

 species of migratory birds, breeding can as a rule be only safely 

 accomplished in a given area ; and farther, that during the 

 greater part of the rest of the year sufficient food cannot be 

 obtained in that area. It will follow that those birds which do 

 not leave the breeding area at the proper season will suffer, and 

 ultimately become extinct; which will also be the fate of those 



