28 STRUCTUEE AND LITEEATUEE. 



the separation of the forms, exposure to a changing 

 environment has assisted in giving character to the 

 species. Por the results of some very careful studies 

 on the characters of allied species in the North- 

 American and European Noctuidse, the papers of 

 Dr. Speyer in the ' Stettiner ent. Zeitung ' should 

 he consulted. 



Many of the North -American species more or less 

 closely resemhle European insects. There is an 

 almost perfect gradation hetween absolutely undis- 

 tinguishable forms, occurring on both continents, 

 such as Xauthia Silago, to perfectly dissimilar ones. 

 Again, the caterpillars seem to have submitted to 

 independent modification, while the moths produced 

 by them remain comparatively unaltered, e. g. the 

 genus Acronycta (cf. Ann. N. Y. Lyceum N. H. 

 Vol. xi.. Article xxviii). All these facts, and others 

 presented by myself in the ' Bulletin ' of the Buffalo 

 Society of Natural Sciences, point to the arising of 

 species by derivation. When we turn to the ques- 

 tion of the distribution of the forms of Noctuidse, 

 we must look to former geological epochs for most 

 of the explanation. The North- American Noctuidse 

 are evidently descended in great part from a former 

 circumpolar fauna during the Tertiary Period. In a 

 paper read before the American Association for the 

 Advancement of Science in 1875, and printed in Sil- 

 liman's Journal, 3rd series. Vol. x., No. 59,1 brought 

 together facts to show the way in which the Glacial 

 Period has influenced the present distribution of 

 our North-American insects. I also reprint, with 

 this Essay, a more popularly written paper, entitled 



