페이지 이미지

With this preliminary definition of the organic characters, which appear to guide to a conception of the most natural primary groups of the class Mammalia, I next proceed to define the groups of secondary importance, or the subdivisions of the foregoing subclasses.

The Lyencephalous Mammalia are unguiculate: some have the 'optic lobes' simple, others partly subdivided, or complicated by accessory ganglions, the lobes being then called 'bigeminal bodies.'

The Lyencephala with simple optic lobes are 'edentulous' or without calcified teeth, and are devoid of external ears, scrotum, nipples, and marsupial pouch: they are true 'testiconda:' they have a coracoid bone extending from the scapula to the sternum, and also an epicoracoid and episternum as in Lizards: they are unguiculate and pentadactyle, with a supplementary tarsal bone supporting a perforated spur in the male. The order so characterized is called 'Monotremata,' in reference to the single excretory and generative outlet, which, however, is by no means peculiar to them among Mammalia. It includes two genera—Echidna and Ornithorhynchus. Of the first, the species are terrestrial, insectivorous, chiefly myrmecophagous, having the beak-like slender jaws, and long cylindrical tongue of the true anteaters; but they are covered, like the hedgehog, with spines. Of the second genus, the species are aquatic, with a flattened beak, like that of a duck, which is used in the anserine manner to extract insects and worms from the mud: but they are clothed with a close fine fur like that of a mole, whence the name 'duck-mole' by which these anomalous quadrupeds are commonly known to the colonists. Both genera of Monotremes are strictly limited to Australia and Tasmania.

The Lyencephela with divided optic lobes, forming the 'corpora bigemina' and 'quadrigemina'of anthropotomists, have teeth, and with rare exceptions, the three kinds, viz. incisors, canines, and molars. They are called MaesupiaLia, because they are distinguished by a peculiar pouch or duplicature of the abdominal integument, which in the males is everted, forming a pendulous bag, and in the females is inverted, forming a hidden pouch containing the nipples and usually sheltering the young for a certain period after their birth: they have the marsupial bones in common with the Monotremes; a much varied dentition, especially as regards the number of incisors, but usually including 4 true molars; and never more than 3 premolars1 (fig. 2): the angle of the lower jaw (ib. a) is more or less inverted8.

With the exception of one genus, Didelphys, which is American, and another genus Cuscus, which is Malayan, all the known existing Marsupials belong to Australia, Tasmania, and New Guinea. The grazing and browsing Kangaroos are rarely seen abroad in full daylight, save in dark rainy weather. Most of the Marsupialia are nocturnal. Zoological wanderers in Australia, viewing its plains and scanning its scrubs by broad daylight, are struck by the seeming absence of mammalian life; but during the brief twilight and dawn, or by the light of the moon, numerous forms are seen to emerge from their hiding-places and illustrate the variety of marsupial life with which many parts of the continent abound. We may associate with their low position in the mammalian scale the prevalent habit amongst the Marsupialia of limiting the exercise of the faculties of active life to the period when they are shielded by the obscurity of night.

The premature birth of the offspring, and its transference to the tegumentary pouch, in which it remains suspended to the nipple for a period answering to that of uterine life in higher mammals, relate to the peculiarities of the climate of Australia.

The adventurous and much-enduring explorers of that continent bear uniform testimony to the want of water as the

1 Outlinesofa Classification ofthe Marsupialia, Trans.Zool.Soc.Vol.n.i8$<).

2 For other Osteological and Dental characteristics of the Marsupialia, see the paper above cited, and that On the Osteology of the Marsupialia, Trans. Zool. Soc. Vol. n. p. 379 (1838).

chief cause of their sufferings and danger. During the dry- season the rivers are converted into pools, 'few and far between;' and the drought is sometimes continued so long as to dry up these. An ordinary non-marsupial quadruped, such as the wild cat or fox, having deposited her young in the nest or burrow, would in such a climate, at the droughtiest period of her existence, be compelled to travel a hundred, perhaps two hundred miles, in order to quench her thirst. Before she could return her blind and helpless litter would have perished. By the marsupial modification the mother is enabled to carry her offspring with her in the long migrations necessitated by the scarcity of water.

With the climatal peculiarities of Australia, therefore, we may connect the peculiar modifications of those members of the mammalian class which are most widely distributed over that continent. But the principle of final causes receives more especial illustrations from the contingent particulars of the marsupial organization. The new-born Kangaroo is an inch in length, naked, blind, with very rudimental limbs and tail: in one which I examined the morning after the birth, I could discern no act of sucking: it hung, like a germ, from the end of the long nipple, and seemed unable to draw sustenance therefrom by its own efforts. The mother, accordingly, is provided with a peculiar adaptation of a muscle (cremaster) to the mammary gland, by which she can inject the milk from the nipple into the mouth of the pendulous embryo. Were the larynx of the little creature like that of the parent, the milk might—probably would—enter the windpipe and cause suffocation: but the foetal larynx is cone-shaped, with the opening at the apex, which projects, as in the whale-tribe, into the back aperture of the nostrils, where it is closely embraced by the muscles of the 'soft palate.' The air-passage is thus completely separated from the fauces, and the injected milk passes in a divided stream on either side the base of the larynx into the oesophagus. These correlated modifications of maternal and fcetal structures, designed with especial reference to the peculiar conditions of both mother and offspring, afford, as it seems to me, irrefragable evidence of Creative foresight.

The Lissencephala, or smooth-brained placental Mammalia, form a group which I consider as equivalent to the Lyencephala or Implacentals; and which includes the following orders, Rodentia, Insectivora, Cheiroptera and Bruta.

The Rodentia are characterized by two large and long curved incisors in each jaw, separated by a wide interval from the molars; the teeth being so constructed, and the jaw so articulated, as to effect the reduction of the food to small particles by acts of rapid and continued gnawing, whence the name of the order. The orbits are not separated from the temporal fossae. The male glands pass periodically from the abdomen into a temporary scrotum, and are associated with prostatic and vesicular glands. The placenta is commonly discoid, but is sometimes a circular mass (Cavy), or flattened and divided into three or more lobes (Lepus). The Beaver and Capybara are the giants of the order, which chiefly consists of small, numerous, prolific and diversified unguiculate genera, subsisting wholly or in part on vegetable food. Some Rodents, e.g. the Lemmings, perform remarkable migrations, the impulse to which, unchecked by dangers or any surmountable obstacles, seems to be mechanical. Many Rodents build very artificial nests, and a few manifest their constructive instinct in association. In all these inferior psychical manifestations we are reminded of Birds. Many Rodents hibernate like Reptiles. They are distributed over all continents. About two-thirds of the known species of Mammalia belong to the Rodent order.

The transition from the Marsupials to the Rodents is made by the Wombats; and a transition from the Marsupials is made, by an equally easy step, through the smaller Opossums to the Insectivora. This term is given to the order of small smooth-brained Mammals, the molar teeth of which are bristled with cusps, and are associated with canines and incisors: they are unguiculate, plantigrade, and pentadactyle, and they have complete clavicles. Like Rodents, they are temporary testiconda, and have large prostatic and vesicular glands: like most other Lissencephala, the Insectivora have a discoid or cup-shaped placenta. They do not exist in South America and Australia; their office in these continents is fulfilled by Marsupialia; but true Insectivora abound in all the other continents and their contiguous islands.

The order Cheiroptera, with the exception of the modification of their digits for supporting the large webs that serve as wings, repeat the chief characters of the Insectivora: a few, however, of the larger species are frugivorous and have corresponding modifications of the teeth and stomach. The mammae are pectoral in position.

The most remarkable examples of periodically torpid Mammals are to be found in the terrestrial and volant Insectivora. The frugivorous Bats differ much in dentition from the true Cheiroptera, and would seem to conduct through the Colugos or Flying Lemurs, directly to the Quadrumanous order. The Cheiroptera are cosmopolitan.

The order Bruta, called Edentata by Cuvier, includes two genera (Myrmecophaga and Manis) which are devoid of teeth; the rest possess those organs, which, however, have no true enamel, are never displaced by a second series, and are very rarely implanted in the premaxillary bones. All the species have very long and strong claws. The ischium as well as the ilium unites with the sacrum; the orbit is not divided from the temporal fossa. The Three-toed Sloths {Bradypus) manifest their affinity to the oviparous Vertebrata by the supernumerary cervical vertebra? supporting false ribs and by the convolution of the wind-pipe in the thorax; and I may add that the unusual number—three and twenty pairs—of ribs, forming a very long dorsal, with a short lumbar, region of the spine, in the Two-toed Sloth (Cholcepus), recalls a lacertine structure. The same tendency to an inferior type

« 이전계속 »