454 LAWS OF MULTIPLICATION. 



Perhaps the best examples are supplied by vertebrate 

 animals, and especially those that are most familiar to us. 

 Comparisons between Fishes are unsatisfactory, because of 

 our ignorance of their histories. In some cases Fishes equal 

 in bulk produce widely different numbers of eggs; as the 

 Cod which spawns millions at once, and the Salmon by 

 which nothing like so great a number is spawned. But then 

 the eggs are very unlike in size ; and if the ovaria of the two 

 fishes be compared, the difference between their masses is 

 comparatively moderate. There are, indeed, contrasts which 

 seem at variance with the alleged relation; as that between 

 the Cod and the Stickleback which, though so much smaller, 

 produces fewer ova. The Stickleback's ova, however, are 

 relatively large ; and their total bulk bears as great a ratio to 

 the bulk of the Stickleback as does the bulk of the Cod's ova 

 to that of the Cod. Moreover if, as is not improbable, the 

 reproductive age is arrived at earlier by the Stickleback than 

 by the Cod, the fertility of the species may be greater not- 

 withstanding the smaller number produced by each indi- 

 vidual. Evidence which admits of being tolerably 

 well disentangled is furnished by Birds. They differ but 

 little in their grades of organization; and the habits of life 

 throughout extensive groups of them are so similar, that 

 comparisons may be fairly made. It is true that, as here- 

 after to be shown, the differences of expenditure which 

 differences of bulk entail, have doubtless much to do with 

 the differences of fertility. But we may set down under the 

 present head some of those cases in which the activity, being 

 relatively slight, does not greatly interfere with the relation 

 we are considering; and may note that among such birds 

 having similarly slight activities, the small produce more eggs 

 than the large, and eggs that bear in their total mass a 

 greater ratio to the mass of the parent. Consider, for ex- 

 ample, the gallinaceous birds; which are like one another 

 and unlike birds of most other groups in flying comparatively 

 little. Taking first the wild members of this order, which 



