EXPENDITURE AND GENESIS. 471 



in nutriment; and as motion through the air requires more 

 effort than motion on the ground, this geometrical progression 

 tells more rapidly on Birds than on Mammals. Be this as it 

 may, however, these contrasts support the argument; as do 

 various others which may be set down. The Finch-family, for 

 example, have broods averaging about 5 in number, and have 

 commonly 2 broods in the season; while in the Crow-family 

 the number of the brood is on the average less, and there is 

 but one brood in the season. And then on descending to such 

 small birds as the Wrens and the Tits, we have 8, 10, 12 to 

 15 eggs, and sometimes two broods in the year. One of the 

 best illustrations is furnished by the Swallow-tribe, through- 

 out which there is little or no difference in mode of life or in 

 food. The Sand-Martin, much the least of them, has 4 to 6 

 eggs and two broods ; the Swallow, somewhat larger, has 4 or 

 5; and the Swift (similar in habits though unrelated), larger 

 still, has but 2. Here we see a lower fertility associated 

 in part with greater size, but associated still more con- 

 spicuously with greater expenditure. For the difference of 

 fertility is more than proportionate to the difference of bulk, 

 as shown in other cases ; and for this greater difference there 

 is the reason, that the Swift has to support not only the cost 

 of propelling its larger mass through the air, but also the 

 cost of propelling it at a higher velocity. 



Omitting much evidence of like nature, let us note that 

 disclosed by comparisons of certain groups of birds with 

 other groups. " Skulkers " is the descriptive title applied to 

 the Water-Kail, the Corn-Crake, and their allies, which evade 

 enemies by concealment — consequently expending but little 

 in locomotion. These birds have relatively large broods — 6 

 to 11, 8 to 12, &c. Not less instructive are the contrasts be- 

 tween the Gallinaceous Birds and other Birds of like sizes but 

 more active habits. The Partridge and the Wood-Pigeon are 

 about equal in bulk and have much the same food. Yet while 

 the one has from 12 to 20 young ones, the other has but 2 

 young ones twice a-year : its annual reproduction is less than 



