On the Seasonal Dimorphism of Butterflies. 83 



Lubbock 3 is undoubtedly correct when, for cases 

 like that just mentioned, he attempts to derive 

 alternation of generations from metamorphosis. 

 But if we exclude heterogenesis there still remain 

 a large number of cases of true metagenesis which 

 cannot be explained from this point of view. 



It must be admitted, with Haeckel, that the 

 alternation of generations in the Hydromedusae 

 and Trematoda does not depend, as in the case of 

 Cecidomyia, upon the larvae having acquired the 

 power of nursing, but that the inferior stages of 

 these species always possessed this power which 

 they now only preserve. The nursing Trematode 

 larvae now existing may possibly have been formerly 

 able to propagate themselves also sexually, this 

 mode of propagation having at the present time 

 been transferred to a later phyletic stage. In this 

 case, therefore, metagenesis was not properly pro- 

 duced by metamorphosis, but arose therefrom in 

 the course of the phyletic development, the 

 earlier phyletic stages abandoning the power of 

 sexual reproduction, and preserving the asexual 

 mode of propagation. A third way in which 

 metagenesis might originate is through polymor- 

 phosis. When the latter is combined with 

 asexual reproduction, as is especially the case 

 with the Hydrozoa, metagenesis may be derived 

 therefrom. The successive stages of transfor- 

 mation of one and the same physiological in- 

 3 Loc. cit. chap. iv. 

 G 2 



