58 THE FLIGHT OF BIRDS 
and is busy nesting. The Gannet is known, I 
believe, not to nest tiPhis fifth year; the EHagle 
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. 
