58 THE FLIGHT OF BIRDS 



and is busy nesting. The Gannet is known, I 

 believe, not to nest till his fifth year ; the Eagle 

 comes to maturity later, possibly not till his tenth 

 year. In rate of reproduction, therefore, a most 

 important matter for the species, the small bird has 

 a great advantage. And thus we can account for 

 the small size of birds compared with that of mam- 

 mals without recourse to any such theory as that of 

 Helmholtz. The small bird is a better starter than 

 the big, and he comes to maturity more quickly. 



