82 Studies in the Theory of Descent. 



phyletic series of dissimilar forms, whilst hetero- 

 genesis originates from a phyletic series of simi- 

 lar forms this series, so far as we can at present 

 judge, always consisting of similar sexual genera- 

 tions. The former would thus nearly coincide 

 with Haeckel's progressive, and the latter with his 

 retrogressive metagenesis. Metagenesis may fur- 

 ther originate in various ways. In the first place, 

 from metamorphosis, as for example, in the propa- 

 gation of the celebrated Cecidomyia with nursing 

 larvae. The power which these larvae possess of 

 propagating themselves asexually has evidently 

 been acquired as a secondary character, as appears 

 from the fact that there are many species of the 

 same genus the larvae of which do not nurse, these 

 larvae being themselves undoubted secondary forms 

 produced by the adaptation of this stage of phyletic 

 development to a mode of life widely different 

 from that of the later stages. In the form now 

 possessed by these larvae they could never have 

 represented the final stage of their ontogeny, nei- 

 ther could they have formerly possessed the power 

 of sexual propagation. The conclusion seems 

 inevitable that metagenesis has here proceeded 

 from metamorphosis ; that is to say, one stage 

 of the ontogeny, by acquiring asexual propagation, 

 has changed the originally existing metamorphosis 

 into metagenesis. 



"cyclical propagation." The latter may be well used in con- 

 tradistinction to " metamorphosis." 



