98 Studies in the Theory of Descent. 



phyletically similar generations. But instances 

 are certainly conceivable which present themselves 

 with less clearness and simplicity. In the first 

 place, we do not know whether parthenogenesis 

 may not finally settle down into complete asexual 

 reproduction. Should this be the case, it might 

 be possible that from heterogenesis a mode oi 

 propagation would ultimately arise, which was 

 apparently indistinguishable from pure meta- 

 genesis. Such a state of affairs might result, if 

 the generations settling into asexual reproduction 

 (as, for instance, the plant-lice), at the same time 

 by adaptation to varying conditions of life, under- 

 went considerable change of structure, and 

 entered upon a metamorphosis to some extent 

 retrogressive. We should then be inclined to 

 regard these generations as an earlier phyletic 

 stage, whilst, in fact, they would be a later one, 

 and the idea of metagenesis would thus have been 

 formed after the manner of heterogenesis. 



On the other hand, it is equally conceivable 

 that heterogenesis may have been developed from 

 true metagenesis in the case of larvae which, 

 having acquired the faculty of asexual propaga- 

 tion, are similar in function to sexually mature 

 insects. This possibility is not at first sight ap- 

 parent. If the nursing-larvae of the Cecidomyice 

 were as much like the sexual insects as are the 

 young Orthoptera to the sexually mature forms, 

 we should not know whether to regard them as 



