1864.] 209 



American fauna has been subjected to a searching comparison with 

 that of Europe and other countries by two distinguished European 

 naturalists, Dr. Hagen and Mr. Loew. In the former order it results 

 from Dr. Hagen's investigations, that out of 716 North American spe- 

 cies no less than 16, or 2.23 per cent, are undoubtedly common to 

 Europe and North America, to say nothing of several species of doubt- 

 ful identity, and of 14 North American species which occur also in 

 Asia, Africa or Polynesia.* In the latter Order, Diptera, it results 

 from Mr. Loew's investigations that, out of 2058f North American 

 species or thereabouts, the extraordinary number of 91 species, or 4.42 

 per cent, are ascertained with certainty to be common to Europe and 

 North America, and there are many others which, although they differ 

 slightly in the two countries, are believed by Loew to be of the same 

 descent.| 



But, some will say, all these species may have been introduced into 

 one or the other country, and not be indigenous in both. Mr. Loew 

 investigates this question in the case of Diptera at considerable length, 

 comparing the intermingling of different faunas on the shores of the 

 Mediterranean, where commercial intercourse has been carried on for 

 time immemorial, and not merely for a few centuries, and where the 

 voyages are comparatively brief; and finally decides that it is " utterly 

 improbable that all the species, now occurring on both continents, 

 should have been gradually carried over from one to the other." In 

 the case of the Pseudoneuropterous Dragon-flies, no less than nine spe- 

 cies of which occur both in the New and in the Old World, it is alto- 

 gether out of the question, in view of the well known difficulty of 

 breeding these insects in confinement, that they could have been intro- 

 duced from one country to the other by human agency. 



A strenuous disciple of Prof. Agassiz observes to me, that '■ the most 

 that can be said of those species which are asserted to be common to 



■•■• Hagen's Si/nopsis N. A. Neur. p. 332. 



f Osten Sacken's Catalogue of described N. A. Diptera, contains 2058 species. 

 Very many of these are jjrofessedly mere synonyms ; but on the other hand 

 many new species have been described since that Catalogue appeared (A. D. 

 1858,) and several undescribed species are taken into the account by Loew. 



X Diptera of the Amber-fauna, by Director Loew ; translated in Silliman'.'i 

 Journal, May, 186i, by Baron Osten Sackeu. 



