Order XI. INSECTIVORA.
The Insectivora[[365]] are an order of mammals to which it is (to quote Professor Huxley) "exceedingly difficult to give a definition." They are, however, none of them large animals, and most of them are nocturnal in habit—two circumstances which may have had something to do with their survival from past ages, as may have also their modification to so many and diverse modes of life; for everything points to the antiquity of the group. They are, for instance, more or less plantigrade. The snout is generally long, and is often prolonged into a short proboscis.[[366]] There is a tendency for the teeth to be of a generalised type, and their number is often the typical mammalian forty-four. Moreover, trituberculate teeth, which are certainly an ancient form of tooth, are common; and indeed the Insectivora of the southern regions of the globe, e.g. Centetidae, Solenodontidae, and Chrysochloridae, have the most prevalent trituberculism, a fact which is of importance in considering the age of the animal life of these regions of the world. The limbs are, as a rule, provided with five digits. The hemispheres of the brain are usually smooth, and do not extend over the cerebellum. The palate is often fenestrate as in the Marsupials, and as in that group the lower jaw is sometimes inflected. But the latter character also occurs in the Sea-lions and elsewhere. Clavicles are present, as a rule, but not in Potamogale.
There is, furthermore, a distinct tendency towards a disappearance of functional milk teeth, which is best seen in Sorex, where there are only seven milk teeth, none of which ever cut the gum. This suppression of the milk dentition is like that of the Marsupials, Edentates, and Whales, all of which appear to be—the first certainly are—ancient forms of mammalian life.
There is also a fairly well-defined, though shallow, cloaca in many genera. Finally, the testes are purely abdominal in some, and in none is there a full descent into a scrotum, as in the more highly-developed Eutheria.
Sub-Order 1. INSECTIVORA VERA.
Fam. 1. Erinaceidae.—This family contains the genera Erinaceus, Hylomys, and Gymnura.
Hylomys, considered by Dobson to fall within Gymnura, is kept separate by Leche.[[367]] H. suillus is a Malayan animal, small in size, about 5 inches long, with a short tail. Like Gymnura it is spineless. The ears are decidedly large and nude. There is one pair of inguinal and one pair of thoracic teats. The colour above is a rusty brown with yellowish-white under parts. The palms and soles are quite naked. In its general form it recalls Tupaia very much more than its own immediate relatives. There is no doubt, however, of its systematic position when the skeleton and teeth are examined. A variety has been described from altitudes of 3000 to 8000 feet on Mount Kina Balu in Borneo. It has the complete dentition of forty-four teeth. There are fourteen pairs of ribs. As in Gymnura the tibia and fibula are united below. The genus is considered by Leche to be the oldest existing type of Erinaceidae.
Gymnura[[368]] is also a Malayan form with the complete dentition of the last, but with fifteen pairs of ribs and a longer tail, consisting of twenty-three vertebrae as against fourteen. There is, as with Hylomys, but one species, G. rafflesii. This animal has a peculiar odour, resembling decomposed cooked vegetables.
The under surface of the tail is rough, and it is thought by Dr. Blanford that it may be of use to the animal in climbing. Its compressed terminal third and the fringe of stiff bristles on the under surface of this indicate, according to Dr. Dobson, powers of swimming, or at any rate a not very remote ancestry of swimming creatures. It is purely insectivorous in diet.