144 THE INDUCTIONS OF BIOLOGY. 



to similar conditions of life, some proportion usually obtains 

 between the sizes of the ova and the sizes of the adult indi 

 viduals; though in the cases of the sturgeon and the tunny 

 there are exceptions, probably determined by the circum 

 stances of oviposition and those of juvenile life. Eeptilcs 

 have eggs that are smaller in number, and relatively greater 

 in mass, than those of fishes; and throughout this class, too, 

 there is a general congruity between the bulk of the egg and 

 the bulk of the adult creature. As a group, birds show us 

 further limitations in the numbers of their eggs as well as 

 further increase in their relative sizes ; and from the minute 

 eggs of the humming-bird up to the immense ones of the 

 Epiornis, holding several quarts, we see that, speaking gene 

 rally, the greater the eggs the greater the birds. Finally, 

 among mammals (omitting the marsupials) the young are 

 born, not only of comparatively large sizes, but with ad 

 vanced organizations; and throughout this sub-division of 

 the Vertebrata, as throughout the others, there is a mani 

 fest connexion between the sizes at birth and the sizes at 

 maturity. As having a kindred meaning, there must 



linally be noted the fact that the young of these highest 

 animals, besides starting in life with bodies of considerable 

 sizes, almost fully organized, are, during subsequent periods 

 of greater or less length, supplied with nutriment in birds 

 by feeding and in mammals by suckling and afterwards by 

 feeding. So that beyond the mass and organization directly 

 bequeathed, a bird or mammal obtains a further large mass 

 at but little cost to itself. 



Were exhaustive treatment of the topic intended, it would 

 be needful to give a paragraph to each of the incidental cir 

 cumstances by which growth may be aided or restricted : 

 such facts as that an entozoon is limited by the size of the 

 creature, or even the organ, in which it thrives; that an 

 epizoon, though getting abundant nutriment without appre 

 ciable exertion, is restricted to that small bulk at which it 

 escapes ready detection by the animal it infests; that some- 



