136 REPRODUCTION. 



SECTION III. 



CONSEQUENCES OF ALTERNATE REPRODUCTION. 



351. These various examples of alternate reproduction 

 render it evident, that this phenomenon can be no longer 

 considered as an anomaly in Nature ; but as the plan for 

 advancing those animals in which it occurs to the highest 

 point of perfection of which they are susceptible. More- 

 over, it has been noticed among all classes of invertebrated 

 animals ; while among the Vertebrates it is as yet unknown. 

 It would seem that the individual life of the lower animals 

 has not force enough to pass continuously, and, as it were, 

 with one stride, through all the phases of its development ; 

 but, in order to accomplish this, it must either be born in a 

 new form, as in the case of alternate reproduction, or un- 

 dergo metamorphoses, which are a sort of second birth. 



352. Many analogies may be discovered between alter- 

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

 mal produced from a nurse afterwards transformed into a 

 Distoma, by undergoing a regular metamorphosis. 



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 higher Polyps (the Coryne), the third generation begins to disengage 

 itself. Among some of the intestinal worms (the Distoma), the third gen- 

 eration 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. 



