450 LAWS OF MULTIPLICATION. 



effort than motion on the ground, this geometric :1 progression 

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

 may, however, these contrasts suj)port the argument ; as do 

 various others that may be set down. The Finch family, foi- 

 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 a season. And then on descending to such 

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

 J 5 eggs, and often two broods in the j^ear. One of the best 

 illustrations is furnished b}^ the Swallow-tribe, throughout 

 which there is little or no difference in mode of life or in food. 

 The Sand- Martin, much the least of them, has usually 6 eggs ; 

 the Swallow, somewhat larger, has 4 or 5 ; and the Swifc, 

 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 then 

 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-Rail, 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, «S: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 10 to 15 young ones, the other has but 2 

 young ones twice a-year ; its annual reproduction is but 

 one-third. It may be said that the ability of the Partridge 

 to bring up so large a brood, is due to that habit of its tribe 



