348 BEPRODTJCTION. 



SECTION III. 



CONSEQUENCES OF ALTERNATE GENERATION. 



533. THESE various examples of alternate generation render 

 it evident, that this phenomenon ought not to be considered 

 as an anomaly in nature ; but as the special plan of develop- 

 ment, leading those animals in which it occurs to the highest 

 degree of perfection of which they are susceptible. Moreover, it 

 has been noticed among all types of the invertebrated animals ; 

 while among the vertebrata it is as yet unknown. It would 

 seem that individual life in the lower animals is not denned 

 within such precise limits as in the higher types, owing, perhaps, 

 to the greater uniformity and independence of their consti- 

 tuent elements, the cells ; and that instead of passing at one 

 stride, as it were, through all the phases of their development, 

 in order to accomplish it, they must either be born in a new 

 form, as in the case of alternate generation, or undergo meta- 

 morphoses, which are a sort of second birth. 



534. Many analogies may be discovered between alter- 

 nate reproduction and metamorphosis. They are parallel 

 lines leading to the same end, namely, the development of 

 the species. Nor is it rare to see them coexisting in the same 

 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. 



535. 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 produces 

 the Cercaria is manifestly an inferior state, just as the chry- 

 salis is inferior to the butterfly. 



production has been observed, we find that the progress displayed in each 

 type consists precisely in the increasing freedom of the individual in its 

 various forms. At first, we have all the generations united in a common 

 trunk, as in the lower polyps and in plants ; then in the Medusa and in some 

 of the hydraform polyps (the Coryne), the third generation hegins to disen- 

 gage itself. Among some of the intestinal worms (the D-istoma), the third 

 generation is enclosed within its nurse, and this in its turn is contained 

 in the body of the grand nurse, while the complete Distoma lives as a 

 parasitic worm in the body of other animals, or even swims freely about 

 in the larva state, as Cercaria. Finally, in the plant-lice, all the genera- 

 tions, the nurses as well as the perfect animals, are separate individuals. 



