Family 9. MYOGALIDÆ.
Genera—Myogale, Urotrichus, Uropsilus.
Family 10. SORICIDÆ.
Genera—Sorex, Blarina, Crocidura, Crossopus, Nectogale, Anurosorex.
Only in one respect have we thought it desirable to depart from Professor Mivart’s system, namely, in raising the Desmans (Myogalidæ) to the rank of a distinct family. This course was adopted for the sake of simplicity in the classification, as the combination of characters presented by those animals places them so remarkably between the Moles and the Shrews, that from a zoological point of view they cannot satisfactorily be referred to either.
One thing that will strike the reader at once is the great number of family types, for the most part strongly characterised, that can be distinguished in so small an order. Mr. Wallace estimates the total number of species of Insectivora at 135, and of these about 65, or nearly one-half, belong to the single family of the Shrews, leaving about 70 species for all the other families; and of these 34 species, or again nearly one-half, are referred to the two widely distributed groups the Hedgehogs and the Moles.
Considering these facts, and the clear differentiation of most of the forms, notwithstanding the existence of those types already alluded to, which in several of the families seem to lead towards the Soricidæ, we can hardly avoid agreeing with Mr. Wallace in regarding the existing Insectivora as “the detached fragments of a much more extensive group of animals, now almost extinct,” a view which is strongly corroborated by the geographical distribution of the animals.
Curiously enough several of the smaller and more peculiar families are limited much in the same way as the Pteropine Bats and Lemurs, chiefly to the countries surrounding the great Indian ocean, beneath which, as we have already stated, the hypothetical continent of Lemuria is very probably submerged. The Galeopithecidæ and Tupaiidæ are almost confined to the Malayan region, and the Centetidæ (with the exception of the anomalous genus Solenodon) are peculiar to Madagascar; the Macroscelididæ have their home on the eastern coast of Africa, except a single species which occurs in the northern part of that continent; the Chrysochloridæ are exclusively South African; and the curious Potamogale inhabits some of the West African rivers. Thus, except in the case of Solenodon, the whole of these groups are now represented solely within the region inhabited by the Pteropine Bats. Does this point to a “Lemurian” origin, or at any rate to a great former development in the Lemurian land, of the Insectivorous Mammalia?
Of the more widely distributed families, the Erinaceidæ occur chiefly in the northern temperate regions of the Eastern hemisphere, stretching away continuously from Europe and the North African deserts, through Asia Minor and Persia, and across Central Asia to the Pacific Coast, whilst one or two species occur in South Africa, and one very aberrant form, the Bulau (Gymnura), is found in the Malayan region, along with the Bangsrings, to which it is allied through the genus Hylomys. The true Moles and the Shrews occur in the northern parts of both hemispheres, and the latter family, indeed, is represented in all parts of the world except South America and the Australian region. The Desmans, which stand in so peculiar a position between the Shrews and the Moles, present a curious instance of what has been called “discontinuous distribution,” the two nearly allied species being found only in two localities, separated from each other by the whole breadth of the European continent. The entire absence of Insectivora from the South American continent, and the presence of the Solenodons, which seem to be most nearly related to the Centetidæ of Madagascar, in Cuba and St. Domingo, are further remarkable facts in the geographical distribution of these animals. Scarcely less singular is the distribution of the two species of Urotrichus, one of which occurs in Japan, and the other on the Pacific coast of North America.
The evidence derived from the fossil remains of Insectivora, as to the former history of the order, in its bearing upon the present geographical distribution of its members, is very inconclusive; but the principal facts to be gathered from it is that from Miocene times to the present day the representatives of the order in different localities, so far as these are known, have generally belonged to the same types, and no undoubted remains of Insectivora are known from earlier formations than the Miocene. At one time, indeed, some of the beautiful Mammalian fossils of the Stonesfield slate (Lower Oolite) of Oxfordshire were regarded as probably representing Insectivora, but their Marsupial character is now generally recognised; and this is the case also with the Dromotherium from the Trias of North Carolina, which was at one time believed to carry the present order so far back in time.