470 LAWS OP MULTIPLICATION. 



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 through 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 Eooks 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 3, some- 

 times only 1. As we descend to the Kites and Falcons, the 

 number is 2 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 impossible 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 locomo- 

 tion. 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 the 

 surfaces of digestive systems vary as the squares, the expense 

 of flight increases more rapidly than does the ability to take 



