CONSEQUENCES OF ALTERNATE REPRODUCTION. 137 



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 spe- 

 cies, but the cycle of development, instead of being accom- 

 plished in a single individual, would involve two or more 

 generations. 



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

 deriving the character of a species from the sexual forms 

 alone, namely, the male and the female, is not always satis- 

 factory ; since there is a large number of animals 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 sepa- 

 rate individuals, the grand-nurse, the nurse, the larva or 

 Cercaria, and the Distoma, in which the sexes are not sepa- 



