436 LAWS OF MULTIPLICATION. 



the degrees of fertility, if measured by the numbers of 

 fertilized germs deposited, are extremely unlike, they arc 

 less unlike if measured by the numbers of young which arc 

 hatched and survive long enough to take care of themselves ; 

 nor will the tax on the parent-Cod seem so immensely dif 

 ferent from that on the parent-Amis, if the masses of the 

 ova, instead of their numbers, are compared. Again, 



while sometimes the parental loss is little else but the matter 

 deducted to form eggs, &c., at other times it takes the 

 shape of a small direct deduction joined with a large indirect 

 outlay. The Mason-wasp furnishes a typical instance. In 

 journeyings hither and thither to fetch bit by bit the 

 materials for building a cell; in putting together these 

 materials, as well as in secreting glutinous matter to act as 

 cement ; and then, afterwards, in the labour of seeking for, 

 and carrying, the small caterpillars with which it fills up the 

 cell to serve its larva with food when it emerges from the 

 egg; the Mason- wasp expends more substance than is con 

 tained in the egg itself. And this supplementary expenditure 

 is manifestly so great that but few eggs can be housed and 

 provisioned. 



Estimates of the cost of Genesis are further complicated 

 by variations in the ratio borne by the two sexes. Among 

 Fishes the mass of milt approaches in size the mass of spawn ; 

 but among higher Vertebi*ata the substance lost by the one 

 sex in the shape of- sperm-cells is small compared with that 

 lost by the other sex in the shape of albumen stored-up in 

 the eggs, or blood supplied to the foetus, or milk given to the 

 young. Then there come the differences of indirect tax 

 on males and females. While, frequently, the fostering of 

 the young devolves entirely on the female, occasionally the 

 male undertakes it wholly or in part. After building a 

 nest, the male Stickleback guards the eggs till they are 

 hatched ; as does also the great Silurus glanis for some forty 

 days, during which he takes no food. And then, among most 

 birds, we have the male occupied in feeding the female during 



