404 



considered, at a time when dimorphism was unknown, as distinct 

 species, are proved to be the same. It encourages us to search for 

 other cases of variation, in which dimorphism may play a part, 

 though not hitherto suspected. 



Mr. Wallace considers as " analogous" to dimorphism, though 

 " not of an identical nature " with it, the phenomenon of the dis- 

 similarity of the spring and autumnal broods in many species of 

 the Lepidoptera which have two broods in the year. It is one that 

 a local entomologist may investigate with advantage. He can 

 ascertain what species there are in his neighbourhood having this 

 habit; observe whether the differences in the two broods are 

 constant from year to year ; whether they vary according to the 

 character of the season ;* whether they are found in both sexes 

 alike ; whether there is any difference in the food of the larvae of 

 the two broods, or other circumstances that throw light upon the 

 question. 



Leaving the subject of variation, I proceed to notice another 

 matter for inquiry likely to yield important results, if taken up by 

 the local entomologist, namely, all that relates to the reproduction 

 of insects. Many anomaUes in this part of their history have been 

 noticed of late years, and there are probably others as yet unob- 

 served, showing how imperfect our knowledge is on this subject, 

 and how mistaken were some of the views formerly entertained. 

 It was at one time generally supposed that among insects, as is 

 universally the case among vertebrates, there could be no con- 

 tinuance of the species without the united influence of the two 

 sexes. And no doubt this is the general rule, but there are many 

 exceptions to it. The phenomena now comprised under the names 

 of parthenogenesis, pseudo-parthenogenesis, and metagenesis, lead 

 us to the knowledge of other ways in which " the production of new 

 organisms is carried on," and involve considerations that tend to 

 enlarge our views respecting development. 



We have an example of pseudo-parthenogenesis in the case of 



• The occasionally dwarfed size of certain species of butterfly has been 

 attributed in some instances to peculiarly hot seasons, as in 1868. — If this ba 

 correct, it shows that seasons do exercise an influence according to their cha- 

 racter. See Ent. Trans. 1868. Proceed., xxxviii. 



