The leading peculiarities of the Insectivora may be briefly indicated, with reference to the groups which approach them most closely in certain points of structure. The limbs are all organised for walking or digging, the fore limbs never being modified, as in the Bats, into organs of flight, and the two bones of the fore-arm (radius and ulna) are always more or less distinct. There is no opposable thumb, either on the fore or the hind feet. The teeth, which are always encased in enamel, are of the usual three kinds—incisors, canines, and molars[251]—and the dentition generally resembles that of the strictly Insectivorous Bats, the molars especially being similarly furnished with several sharp cusps or points, which are regarded as characteristic of Insect-eating Mammals. All the teeth are implanted in the jaws by roots.
In the development of the tail, and the nature of the covering of the skin, the Insectivora present considerable diversities, which will be referred to hereafter. Their feet generally consist of five toes, all armed with claws, and nearly all are plantigrade—that is to say, they apply the whole, or nearly the whole, of the sole of the foot to the ground in walking. With a single exception (Potamogale, which is rather anomalous in some other respects), all the Insectivora are provided with complete clavicles, or collar-bones—a character which serves to distinguish them from the Carnivora, in which the collar-bones are either deficient or imperfectly developed. The teats are generally numerous, and situated on the abdomen, the only exceptions being the anomalous Colugo, or so-called Flying Lemur, and the Golden Moles, in which the teats are situated on the breast.
Zoologists are now pretty well agreed as to the classification of these animals, although there are still differences of opinion as to the best arrangement of the families, and some minor points. The classification here adopted is founded upon that proposed by Professor Mivart in 1871, and afterwards modified by Professor Theodore Gill. In this the whole order is divided into nine families, the first of which is so anomalous, and so divergent from all the rest in its characters, as to have led to its being treated as constituting a distinct sub-order (Dermoptera).
FAMILY I.—GALEOPITHECIDÆ, OR COLUGOS.
The animals which constitute this family, now regarded as constituting only two species (although the right even of one of these to specific rank is somewhat doubtful), are in truth amongst the most anomalous of Mammals. In their characters they present the most singular resemblances to at least three orders of Mammalia, in which they have been successively placed by various zoologists. Discovered by the Dutch voyagers of the seventeenth century in the luxuriant forests of the Eastern islands, their general Lemur-like aspect led the naturalists of those days to class them with those creatures, and Camelli, the distinguished botanist, gave them the name of Galeopithecus, which became in Petiver’s hands, “Cato-simius volans,” or the Flying Cat-Monkey. Seba left out the Monkey, and called the animal simply the Flying Cat of Ternate (Felis volans ternatea); whilst Bontius, laying undue weight on its so-called flying powers, regarded it as a Bat, and gave it the name of Vespertilio admirabilis. Linnæus accepted the Lemur hypothesis, and placed the animal in his genus Lemur, under the name of Lemur volans, or the Flying Lemur, and this position it continued to hold for a very long time, although Pallas separated it from the true Lemurs under Camelli’s name of Galeopithecus. No one ever reverted to the notion that the Colugo was a Bat, but from time to time various naturalists have pointed out that in many of its characters it approached the Insectivora; and of late years the evidence in favour of its belonging to that order has been put forward so strongly, that nowadays nearly all zoologists regard it as an exceedingly aberrant member of the group, with more or less distinct tendencies towards the Bats and the Lemurs, and perhaps with some faint trace of the Marsupial about it. Mr. Wallace, speaking, of course, from the standpoint of the theory of evolution, says that “this animal seems, in fact, to be a lateral offshoot of some low form, which has survived during the process of development of the Insectivora, the Lemuroidea, and the Marsupials, from an ancestral type.” There is no doubt that the beast is sufficiently dissimilar from all other known Mammals to give a considerable air of probability to the assumption of its being a survivor from some earlier period of the earth’s history; but as it is here we must do the best we can with it, and its natural position is certainly between the true Insectivora and the Lemurs. As the characters of the family are founded virtually upon a single species, one description will serve.
THE COLUGO, OR FLYING LEMUR.[252]
The species known to the older naturalists is found in Malacca, Sumatra, and Borneo, where it inhabits the forests, climbing the trees like a Squirrel by the aid of its claws, and passing through the air from one tree to another by means of a membrane (patagium), which extends along the sides of the body, and can be stretched by the extension of the limbs to which it is attached so as to act as a sort of parachute, which supports its owner after the same fashion as the very similar fold of skin that exists in the same position in the so-called Flying Squirrels and Flying Opossums. In the Colugo, however, this curious arrangement is carried further than in the other groups of Mammals just mentioned; for, as in the Bats, there is a distinct antebrachial membrane, stretching along the front of the arms from the wrists to the sides of the neck; and the space between the hind limbs is occupied by an ample triangular membrane, down the middle of which the long tail passes, and which is also stretched by the extension of the limbs. Even the toes are joined by membranes as far as the base of the claws, and this great development of the skin must be regarded as to a certain extent approximating the creature to the Bats. The whole of this fold of skin is clothed both above and beneath with hair; and although some observers have described the animal as moving its expanded membranes during flight, no approach to the peculiar action of the Bat’s wing can ever be made by it. The most striking point in which it exceeds the other parachute-bearing Mammals is the development of the membrane between the hind limbs, and this, by the action of the tail, may be made to exert a powerful influence upon the course of the animal during its so-called flights. Mr. Wallace, who had the opportunity of observing the Colugo in its native haunts, describes its flight as follows:—“Once, in a bright twilight,” he says, “I saw one of these animals run up a trunk in a rather open place, and then glide obliquely through the air to another tree, on which it alighted near its base, and immediately began to ascend. I paced the distance from the one tree to the other, and found it to be seventy yards, and the amount of descent I estimated at not more than thirty-five or forty feet, or less than one in five. This, I think, proves that the animal must have some power of guiding itself through the air, otherwise in so long a distance it would have little chance of alighting exactly upon the trunk.” In a subsequent work, following other writers, he refers this power to the agency of the tail, and even thinks that the animal may rise over obstacles in its course by the elevatory action of that organ. The tail is of considerable length, and according to some writers its extremity has a slight prehensile action which is of assistance to the animal in climbing. The membranes, when not in use, as when the Colugo is walking or climbing, fall in great folds at the sides of the body.
Passing now, by a natural transition, from the parachute-like membranes to the limbs which traverse and serve to extend them, we find that these exhibit certain peculiarities of structure which are amongst the anomalies of this singular creature. The bones of both fore and hind limbs are elongated and slender—a character which contrasts strongly with the general state of things in the Insectivora—and the ulna, which is particularly slender, is united to the radius towards the extremity. The feet consist of five digits, and they are specially adapted to enable the animal to climb readily upon the bark of the trunks and branches of trees. In the hind feet especially part of the tarsal bones (the navicular and cuboides) are constructed so that they can easily turn upon the astragalus and calcaneum, and thus the sole is turned inwards, an arrangement which facilitates the clasping action of the feet. The inner digits in all the feet possess considerable power of independent motion, although they are never converted into opposable thumbs; and this arrangement, combined with the presence of sharp strong claws upon all the toes, must greatly favour the peculiar mode of life of the animal. It is to be remarked that the structure of the hind feet presents some analogy to that prevailing in Bats, and that in repose the Colugo suspends itself from a branch by the fore and hind feet, with the body and head hanging downwards, which is also a habit somewhat reminding us of the Chiroptera.
The head in the Galeopithecus is tolerably broad and a little flattened; the eyes are placed more laterally than in the Lemurs, and the orbits containing them form a bony ring which is interrupted behind.
