168 REPRODUCTION. 



animal. Thus, in the Cercaria, we have seen an animal pro- 

 duced from a nurse afterwards transformed into a Distoma, 

 by undergoing a regular metamorphosis. 



353. In each new generation, as in each new metamor- 

 phosis, a real progress is made, and the form which results 

 is more perfect than its predecessor. The nurse that pro- 

 duces the Cercaria is manifestly an inferior state, just as the 

 chrysalis is inferior to the butterfly. 



354. But there is this essential difference between the 

 metamorphoses of the caterpillar and alternate reproduction, 

 that, in the former case, the same individual passes through 

 all the phases of development ; whereas, in the latter, the 

 individual disappears, and makes way for another, which 

 carries out what its predecessors had begun. It would give 

 a correct idea of this difference to suppose that the tadpole, 

 instead of being itself transformed into a frog, should die, 

 having first brought forth young frogs ; or that the chrysalis 

 should, in the same way, produce young butterflies. In 

 either case, the young would still belong to the same species, 

 but the cycle of development, instead of being accomplished 

 in a single individual, would involve two or more acts of 

 generation. 



355. It follows, therefore, that the general practice of de- 

 riving the character of a species from the sexual forms alone, 

 namely, the male and the female, is not applicable to all 

 classes of animals ; since there are large numbers whose 

 various phases are represented by distinct individuals, en- 

 dowed with peculiarities of their own. Thus, while in the 

 stag the species is represented by two individuals only, stag 

 and hind, the Medusa, on the other hand, is represented 

 under the form of three different types of animals ; the first 

 is free, like the Infusoria, the second is fixed on a stalk, like 

 a polyp, and the third again is free, consisting in its turn 

 of male and female. In the Distoma, also, there are four 



