1891.] ON THE INSECTIVORA OF THE NEW WORLD. 349 



2. Note on the Derivatiou and Distribution of the Insectivora 

 of the New World. By G. E. Dobson, M.A., F.R.S. 



[Eeceived April 24, 189].] 



Of the ten Families into which the Insectivora are divisible, two 

 only^, and these very closely allied, namely Soricidce and TalpidcB, 

 are represented in the New World, and of the first-named family, 

 composed of eleven genera, three genera only — Sorex, Blarina, and 

 Notiosorex (with a single species) — have representatives in that 

 continent, where all are restricted to the Nearctic Region. On the 

 other hand, the closely connected Palsearctic Region includes repre- 

 sentatives of no less than eight genera, nearly four-fifths of the 

 whole. All the species of the two genera inhabiting the American 

 continent belong to the Red-toothed Shrews, and are, in fact, 

 modified forms of either Sorex or of Soriculus, the former common 

 to both the Palsearctic and Nearctic Regions, the latter found only 

 in a limited portion of the north-eastern parts of the Eastern Hemi- 

 sphere, but represented in the Nearctic Region by the species of 

 Blarina. 



While the species of Blarina are characteristic of the Nearctic 

 Region, those of Sorex are, with few exceptions, closely related one 

 to another, so much so as to be, in my opinion, Nearctic local races 

 only of two well-known Paleearctic species, namely S. vulgaris and 

 S. minutus { = S. pygmeeus), of which the former extends to North 

 America, and the latter is represented there by its but slightly mo- 

 dified descendant S. personatus {-=8. cooperi) and its varieties. 



The Nearctic Shrews were therefore evidently derived from the 

 Palsearctic Region, having migrated from thence probably at a 

 comparatively recent period, if we may judge from the fact that the 

 Water-Shrews of the New World are still referable to the genus 

 Sorex, the changes in their bodily structure due to their altered 

 mode of life not having yet advanced nearly so far as we find in the 

 much more highly specialized Water-Shrews (Crossopus) of the Old 

 World. 



Assuming then, as I believe we are entitled to from a consideration 

 of the above-mentioned facts, that the American Shrews were de- 

 rived from the Palsearctic Region, it is only reasonable to suppose 

 that the immigration took place by the shortest route, namely, from 

 west to east. The total absence of the White-toothed Shrews from 

 the Nearctic Region goes far to prove that the place of entrance of 

 the ancestors of the American Shrews from the Asiatic continent 

 must have occurred at some position north of N. lat. .50°, for one 

 species at least of the genus Crocidura extends as far north as tlie 

 region of the Ussuri river ^. However, there is no difficulty in 

 supposing that the entrance took place in the latitude of Behring's 



^ Not taking into aocount the SolenodontidcB, the species of which are limited 

 to the islands Cuba and Hayti. 



^ A tributary of the Amur River, in E. Manchuria. 



Proc. Zool. Soc— 1891, No. XXIV. 24 



