CONSEQUENCES OF ALTERNATE GENERATION. 167 



SECTION III. 



CONSEQUENCES OF ALTERNATE GENERATION. 



351. These various examples of alternate generation ren- 

 der it evident, that this phenomenon ought not to be consid- 

 ered as an anomaly in Nature ; but as the special plan of de- 

 velopment, 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 inverte- 

 brated animals ; while among the Vertebrates it is as yet 

 unknown. It would seem that individual life in the lower 

 animals is not defined within so precise limits as in the 

 higher types ; owing, perhaps, to the greater uniformity and 

 independence of their constituent 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 metamorphoses, which are 

 a sort of second birth. 



352. Many analogies may be discovered between alternate 

 reproduction and metamorphosis. They are parallel lines 

 that lead to the same end, namely, the development of the 

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



first, we have all the generations united in a common trunk, as in the 

 lower Polyps and in plants ; then in the Medusae and in some of the 

 Hydroid Polyps the third generation begins to disengage itself. Among 

 some of the intestinal worms, (the Distoma,) 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 generations, the 

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



