592 



This solution of the problem of the origin of a large proportion of the 

 species of insects of om- North Temperate realm was adopted by the writer 

 in an article in the "American Naturalist".* It is evident thai the distribu- 

 tion of insects is closely dependent on thai of plants, and thai the origin of 

 our insect-fauna was contemporaneous and followed the vicissitudes of our 

 flora. 



The theory advanced by Ileer and others, and still persisted in by 

 authors, of an intercontinental bridge between' the temperate zones of 

 America and Europe and America and Asia, -besides being opposed by 

 geological reasons of weight, is entirely useless in accounting for the present 

 distribution of insect-life in the temperate zone, as there is a marked absence 

 of Asiatic forms on the Pacific coast, and the presence of what few there arc 

 may be accounted for on the hypothesis of their derivation from a Tertiary 

 arctic continent. On the other hand, there are some remarkable features 

 that ally the lepidopterous fauna, at least of the Pacific coast, to that of 

 Europe, and which are wanting in the Eastern or Atlantic province. The 

 presence of the few European temperate forms occurring in temperate 

 America may be explained by the hypothesis of their migration from a Ter- 

 tiary arctic continent. Indeed, the ancestors of the Pacific-coast Rhaphidia, 

 Parnassius, Papilio zolicaon, Epica//ia, and Callarctia, which are true Euro-* 

 pean-Asiatic types, may have inhabited the arctic Tertiary continent, of which 

 Greenland and Spitsbergen are the remains ; and their descendants forced 

 southward have probably lost their foothold in the Atlantic region, and some, 

 like the Sequoia, survived in California and others in Europe. 



Something more than similarity of climate is required to account for the 

 presence of such generic types, common to the western shores alone of two 

 continents; hence community of origin, with high antiquity, and a southward 

 migration of forms not of tropical origin, are the factors needed to work out 

 the problem. 



That something of this sort has taken place in marine animals we know 

 to lie the fact. Certain forms, now supposed to be extinct on the coast 

 of New England and Scandinavia, such as Yoldia arctica Gray (Nuculaport- 

 landica Hitchcock), arc still living in the seas of Greenland and Spitzbergen. 



* July, 1873, vol. vii, 4.">3. Sue also Proceedings of the Boston Society of Natural History for May 

 7, 1-7.:. published January, 1874, and an article entitled "On the Geographical Distribution of the Moths 

 of Colorado", from the Annual Report of the United States Geological and Geographical Survey of the 

 Territories for 1873, p. 543 (1875). 



