GROWTH AND SEXUAL GENESTS. 433 



fertile ova in the same period. The same individual, pro- 

 ducing in ten days forty eggs, developed with the rapidity 

 above cited, this rate, raised to the tenth power, gives one 

 million of individuals from one parent, on the eleventh day 

 four millions, and on the twelfth day sixteen millions, and so 

 on." Ascending from this extreme, the differences 



of organization and activity greatly complicate the inverse 

 variation of fertility and bulk. Bearing in mind, how- 

 ever, that the rate of multiplication depends much less on the 

 number of each brood than on the quickness with which 

 maturity is reached and a new generation commenced, it will 

 be obvious that though Annelids produce great numbers of 

 ova, yet as they do this at comparatively long intervals, their 

 rates of increase fall immensely below that just instanced in 

 the Rotifers. And when at the other extreme we come to 

 the large articulate animals, such as the Crab and the Lobster, 

 the further diminution of fertility is seen in the still longer 

 delay that occurs before each new generation begins to re- 

 produce. 



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 a million 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 



