THE NATURAL HISTORY OF THE M.A MMALIA.* Tars finely illustrated
volume will be as acceptable to the ordinary public as to the biological world. While presenting the latest results of scientific investigation in an accurate and lucid manner, the language, though not altogether untechnical, is sufficiently so to be easily intelligible to the ordinary reader; and dealing as it does with by far the most generally in- teresting division of the animal kingdom, including man himself, ought to be a welcome addition to every library.
The obviously distinctive characteristic of the Mammalia is, of course, the presence of mammary glands, which afford sustenance to the young in a manner absolutely unparalleled in the animal world. With these are correlated, no doubt, though the lines of relation are difficult, in the present state of our knowledge, to trace, the two pairs of limbs, —but these are also found in many reptiles,—the furry covering of the body and the presence of a diaphragm, or midriff muscle, completely separating the thoracis from the abdominal cavity, and serving as the main agent in effecting the process of respiration. It is surprising how, with simple modifications (and occasional suppressions) of these organs, the mammalia are enabled to live on the earth or beneath its surface, to fly in the air like birds, or spend their lives in the ocean like fishes ; to support every variety of climate, and provide for their sustenance amid the most varied and oppo- site conditions of environment. And it is worthy of remark, in this connection, that of all animals, man is the most cosmo- politan, not, however, by modification of his organs or members —or only in a secondary sense—but by the enormous develop- ment of his psychological and intellectual powers, itself due,in large part at least, to his upright position, the opposition of the human thumb, and above all to the possession of language which allows of the accumulation and tradition of experience, and which perhaps was in its beginnings a mere result of some varietal advance in the nervo-muscular mechanism of articula- tion. At what point of the zigzag line of vertebrate ascent the mammalian branch originated, it is scarcely possible to deter- mine save by way of exclusion. It is quite certain that mammals are neither developed birds nor developed fishes. Birds, indeed, may well have been of later origin than the great class of vertebrate animals which attains its crown and glory in man, and may trace its vertebrate ancestry as far back as a point or node next succeeding that when the fishes branched off from the main trunk. The researches of Pro- fessors Huxley and Cope, and of later investigators, among whom Dr. Baur holds a prominent place, have shown that the starting-point of the Mammalia was somewhere between the origins of the Amphibia and the Reptilia. Some exacter locali- sation may become possible with the advance of science, but the mammalian characters of the lowest known mammals, the mono- tremata (duckbill and echidna) are already well pronounced, and must have resulted from a long course of development of which we can scarcely hope even to know more than a few of the terms. Of the whole mass of organised beings, existing and fossil forms constitute a quite infinitesimal proportion, and their study can at the most but enable us, in the roughest and most general way, to outline in broken portions merely the main branches of the great genealogical tree of life. The succession of life on the globe, to a far greater extent than the geological record, has to be made out from a scattered • An Introduction to the Study of MOM71144, Linn and Extinct. By W. H. Flower, C.B., F.R.S., &c., and B. Lyddeker, F.Z.S., Ac, London: A. and C. Black. 1801.
remnant of unpaged, defaced, and often nearly illegible docu- ments, to which only immense patience can give any connected sense at all, at the best evolving mere fragmentary fractions of a text often capable of various readings and interpetations.
Precisely the same order of difficulties obstructs the work of classification, which in these days has become a question of descent and relationship. As the authors rightly point out, there would be no great difficulty in settling a scheme of classification, had we to deal with existing mammalian forms alone. But the advance of palmontological knowledge con- stantly introduces fresh terms into the various series, neces- sitating new arrangements and different collocations ; and no classification can be proposed that is not sure to require more or less modification in the course of a few years, with a liability to revolutionary changes within no very extended period of time. A good example of what stores of luminous facts may remain shut up in the substance of the globe, is presented by Mr. S. H. Beckles's celebrated explorations near Swanage. There, "in a bed of calcareous mud only 40 ft. long, 10 ft. wide, and averaging 5 in. in depth," a truly mar- vellous collection of mammalian remains was found, the study of which carried back with certainty the origin of the class from tertiary to mesoZoic times.
The classification adopted by the authors is, in its main features, that proposed by Professor Huxley. According to this scheme, the known mammalia, recent and fossil, are arranged in three sub-classes,—the Prototberia, now repre- sented by the duckbill and the echidna; the Metatheria, by the marsupials, or pouched animals ; and the Eatheria, by the remaining forms of mammalian life. Objections of various kinds have been taken to this classification which cannot be discussed here. But it may at least be said that no classifica- tion is likely long to remain a satisfactory one the primary divisions of which exhibit such marked diversity of contents as the first two of the above sub-classes compared with the third. We cannot help thinking that the edentata (armadillos, sloths, &c.), the sirenia (manatees, dugongs), and the whales are improperly grouped with the remaining Entheria (ungulates, carnivores, rodents, &c.), though what to do with them is quite another matter.
As a survival of an extremely ancient mammalian form, the platypus was well worthy of the interest excited by Mr. Cald- well's study of the life-history of the only egg-laying mammal known—investigations of which the scientific world still anxiously awaits the conclusion—but from an adaptationist point of view, the duckbill is scarcely more attractive to the biologist than an ordinary vole or water-rat. There can be little doubt that natural selection acts mainly in relation to self-conservation, promoted chiefly among mammals by modi- fications of a common mechanism, which enables vegetable. feeders to escape, and flesh-feeders to inflict, destruction. It is, therefore, among the latter that we must seek for the most striking and instructive instances of adaptation, and, on the whole, the bats and whales may be said to offer the extreme phases of variation of the mammalian plan, enabling the one order to gain their livelihood in the air, the other in the ocean. According to the authors, the bats (one division of which are fruit-feeders) possess true wings ; but these differ altogether from the wings of birds, which are modifications with accessories of the fore-limbs only, while in bats there are no accessories, and the wings consist merely of prolonga- tions of the integument supported mainly by the fore, but in part also by the hind, limbs. The fore-wing of the bat, indeed, is essentially nothing but an ordinary fore- limb, with one of the two arm-bones reduced and the other increased in length, and with the four fingers greatly elongated. The hind-wing is a membranous extension, supported by the scarcely modified but atrophied hind-limb and the posterior part of the body. Note this very simple modification. In the Cetacea, on the other hand, which are much more fish-like in form than the bats are bird-like, the hind-limbs are only present as mere rudimentary bonelets beneath the skin, and the fore-limbs have neither external arm, fore-arm, nor hand, but are reduced to mere paddles covered by integument, but without digits or nails, though the bones of arm and hand are all present, but flattened and otherwise modified,—indeed, of some of the digits the bones are more numerous than in other mammals The Cetacea, from whales to porpoises and dolphins, all suckle their young, with the aid of a muscular apparatus which compresses the milk-reservoir, and forces the milk into the month of the young cetacean while beneath the water, a position in which, of course, the act of sucking could not be performed.
The plan of the work does not admit of the question of the origin and position of man being discussed, save after the briefest fashion. "The essential attributes," it is justly said, "which distinguish Man, and give him a perfectly isolated position among living creatures, are not to be found in his bodily structure." To what, then, is this "isolated position" due ? The author would appear not to claim it, as a result of his superior articulation, combined with the upright position and the opposition of thumb to digits. There seem to be only two possible answers to the question,—one, that which Professors Flower and Lyddeker do not give ; the other, that the mind of man was an accession to, not a derivation from, his perfected structure. It is clear that man is an animal corporeally ; to what extent he is so emotionally, intellectually, or spiritually, men of science are as ignorant as men of no-science, unless they adopt some such theory as that which in the sentence just quoted, does not appear to be accepted.
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