EXPENDITURE AND GENESIS. 449 



First we have the average contrast, already hinted, between 

 the fertility of Birds and the fertility of Mammals. Compar- 

 ing the large with the large and the small with the small, we 

 see that creatures which continually go through the muscular 

 exertion of sustaining themselves in the air and propelling 

 themselves rapidly through it, are less prolific than creatures 

 of equal weights which go though the smaller exertion of 

 moving about over solid surfaces. Predatory Birds have 

 fewer young ones than predatory Mammals of approximately 

 the same sizes. If we compare Rooks with Rats, or Finches 

 with Mice, we find like differences. And these differences are 

 greater than at first appears. For whereas among Mammals 

 a mother is able, unaided, to bear and suckle and rear half- 

 way to maturity, a brood that probably weighs more in pro- 

 portion than does the brood of a Bird ; a Bird, or at least a 

 Bird that flies much, is unable to do this. Both parents have 

 to help ; and this indicates that the margin for reproduction 

 in each adult individual is smaller. 



Among Birds themselves occur contrasts which may be 

 next considered. In the Raptorial class, various species of 

 which, differing in their sizes, are similarly active in their 

 habits, we see that the small are more prolific than the large. 

 The Golden Eagle has usually 2 eggs : sometimes only 1. 

 As we descend to the Kites and Falcons, the number is 2 or 

 or 3, and 3 or 4. And when we come to the Sparrow-Hawk, 

 3 to 5 is the specified number. Similarly among the Owls : 

 while the Great Eagle-Owl has 2 or 3 eggs, the comparatively 

 small Common Owl has 4 or 5. As before hinted, it is im- 

 possible to say what proportions of these differences are due 

 to unlikenesses of bulk merely, and what proportions are due 

 to unlikenesses in the costs of locomotion. But we may fairly 

 assume that the unlikenesses in the costs of locomotion are 

 here the more important factors. Weights varying as the 

 cubes of the dimensions, while muscular powers vary as the 

 squares, the expense of flight increases more rapidly than the 

 size increases ; and as motion through the air requires more 



