OCR Text |
Show 350 ON THE INSECTIVORA OF THE NEW WORLD. [June 2, Strait at a time when the continents were united at that point, have examined specimens of Sorex vulgaris and of S. minutus from higher latitudes, namely, from the banks of the Khatanga and of the Olenek rivers within the Arctic Circle. The Red-toothed Shrews are, in fact, pre-eminently boreal in their distribution, braving the most rigorous climates of the northern parts of both hemispheres, and thinning out quickly, to finally disappear altogether as we advance south. Their limit appears to be a climatic rather than a territorial one : thus their southern extent in the Palaearctic Region may be very correctly stated to be bounded by the isothermal of 60° Fahr.; the few exceptions noticeable, such as the presence of species of Soriculus south of this line on the southern slopes of the Himalayas, being easily accounted for by the fact that these animals are rarely found there at a lower elevation than 6000 feet, where they enjoy a really temperate climate. This explains how it happens that Shrews are wholly absent from South America. Two species only are found in Central America, where they extend as far south as Costa Rica, being, like the species of Soriculus \ stragglers from the north along the high mountains and elevated table-lands, and therefore enjoying, like them, a comparatively temperate climate, their further advance southward being evidently prevented by the long depression which separates the mountains and elevated plateau of Costa Rica from the Andes, and not by the competition of other animals in the Neotropical Region, as writers on geographical distribution would have us believe. The high temperature of the Isthmus of Panama has, in fact, proved as effectual a barrier to these inhabitants of a boreal zone as the low temperature of the ancient northern isthmus between Asia and America was of old to the sun-loving White-toothed Shrews. There cannot be the least doubt that had a sufficient number of individuals of any of the species of White-toothed Shrews effected an entrance into North xAmerica, they would speedily have found their way into the southern part of that continent and thence into South America, and have continued to exist and multiply there. Similar remarks apply to the Talpidce, the species of which are, like those of the Red-toothed Shrews, restricted to the temperate and sub-boreal zones of the Northern Hemisphere, the instances in which there appears to be an exception to this rule, as in the case of two species which are found on the southern slopes and spurs of the Himalayas, being accounted for by the high elevation of the districts which they inhabit. Of the seventeen known species, four only are found in the New World, and these have much the same distribution as the Red-toothed Soricidce, the chief difference noticeable being that none have been found as yet north of the southern parts of the shores of Hudson's Bay nor to the south of Mexico, the high tem- 1 These have hitherto been supposed to be limited to the southern slopes of the Himalayas; but I have recently discovered, in the collection of the Paris Museum, a specimen of Soriculus caudatus from the mountains of Western Fo-Kien, China, so that it is probable that this genus has really its headquarters in countries to the north and north-east of the Himalayas. |