KXPENDITURE AND GENESIS. 451 



which one of its names, " Scrapers," describes ; and to the 

 accompanying habit of the young, which begin to get their 

 own living as soon as they are hatched : so saving the parents' 

 hibour. Conversely, it may be sail that the inability of the 

 Pigeon to rear more than 2 at a time, is caused by the necessity 

 of fetching everything they eat. But the alleged relation 

 holds nevertheless. On the one hand, a great part of the food 

 which the Partridge chicks pick ap, is food which, in their 

 absence, the mother would have picked up : though each chick 

 costs her far less than a young Pigeon costs its parents, yet 

 the whole of her chicks cost her a great deal in the shape of 

 abstinence — an abstinence she can bear because she has to fly 

 but little. On the other hand, the Pigeon's habit of layino' 

 and hatching but two eggs, must not be referred to any fore- 

 seen necessity of going through so much labour in supporting 

 the young, but to a constitutional tendency established by such 

 labour. This is proved by the curious fact that when do- 

 mesticated, and saved from- such labour by artificial feedino-, 

 Pigeons, says Macgillivray, " are frequently seen sitting on 

 eggs long before the former brood is able to leave the nest, so 

 that the parent bird has at the same time young birds and 

 eggs, to take care of." 



§ 350. Made to illustrate the effect of activity on fertility, 

 most comparisons among Mammals are objectionable: other cir- 

 cumstances are not equal. A few, however, escape this criticism. 



One is that between the Hare and the Eabbit. These are 

 closely- allied species of the same genus, similar in their diet 

 but unlike in their expenditures for locomotion. The rela- 

 tivel3'-inert Rabbit has 5 to 8 3'oung ones' in a litter, aild 

 several litters a-year; while the relatively-active Hare has 

 but 2 to 5 in a litter. This is not all. The E/abbit beofins 

 to breed at six months old; but a year elapses before the 

 Kare begins to breed. These two factors compounded, result 

 in a diiference of fertility far greater than can be ascribed to 

 unlikeness of the two creatures in size. 



