COy SEQUENCES OF ALTEBSATE KEPEODUCTION. 349 



536. But there is this essential difference between the meta- 

 morphoses of the caterpillar and alternate reproduction, that 

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

 sot' development; whereas, in the latter, the individual 

 pears, 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. 



537. 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 applicable to all 



-es of animals ; since there are large numbers whose various 



phases are represented by distinct individuals, endowed with 



peculiarities of their own. Thus, while in the stag the species 



is represented by two individuals only, stag and hind, the 



on the other hand, is represented under the form of 



three different types of animals ; the first is free, like the in- 



: ia; 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 separate individuals, the 



grand nurse, the nurse, the larva or Cercaria, and the Distoma, 



in which the sexes are not separate. Among the Aphides the 



number is much greater still. 



538. The study of alternate generation, besides making us 

 better acquainted with the organization of animals, greatly 

 simplifies our nomenclature. Thus, in future, instead of enu- 

 merating the Distoma and the Cercaria, or the Strobila, the 

 Ephyra and the Medusa, as distinct animals belonging to dif- 

 ferent classes and families, only the name first given to one 

 of these forms will be retained, and the rest be struck from 

 the psip-s of zoology, as representing only the transitory phases 

 of the same species. 



539. Alternate generation always pre-supposes several 

 modes of reproduction, of which the primary is invariably by 

 ovulation. Thus we have seen that the polyps, the medusae, 

 the salpse, &c., produce eggs, which are generally hatched 

 within the mother. The subsequent generation, oil the con- 



