76 Oi\ r THE ORIGIN AND [CHAP. 



pillar, instead of changing into one butterfly, produced 

 several, we should have an instance of alternation of 

 generations. Until lately, however, we knew of no 

 such case among insects ; each larva produced one 

 imago, and that not by generation, but by develop- 

 ment. It has long been known, indeed, that there 

 are species in which certain individuals remain always 

 apterous, while others acquire wings. Many entomo- 

 logists, however, regard these abnormal individuals as 

 perfect, though wingless insects ; and therefore I shall 1 

 found no argument upon these cases, although they 

 appear to me deserving of more attention than they 

 have yet received. 



Recently, however, Prof. Wagner 1 has discovered 

 that, among certain small gnats, the larvae do not 

 directly produce in all cases perfect insects, but give 

 birth to other larvae, which undergo metamorphoses 

 of the usual character, and eventually become gnats. 

 His observations have been confirmed, as regards 

 this main fact, by other naturalists ; and Grimm has 

 met with a species of Chironomus in which the pupae 

 lay eggs. 2 



Here, then, we have a distinct case of alternation of 

 generations, as characterized by Steenstrup. Probably 

 other cases will be discovered in which insects undeni- 

 ably in the larval state will be found fertile. Nay, 

 it seems to me possible, if not probable, that some 

 larvae which do not now breed may, in the course of 

 ages, acquire the power of doing so. If this idea is 

 correct, it shows how the remarkable phenomenon, 



1 Zeit. fiirWiss. Zool. 1863. 



2 Mem. de 1'Acad. Imp. de St. Pctersbouro-. vol. xv. 1870. 



