duction of these animals, they are, to a certain extent, working in the dark.

In conclusion, I may sum up the results of this lecture by stating that, in the present state of our knowledge, the whole Animal Kingdom is divisible into eight primary categories or groups, no two of which are susceptible, in the present state of knowledge, of being defined by characters which shall be at once common and diagnostic.

I leave aside altogether the question of the equivalency of these groups; and, as I have already stated, I entertain some doubts regarding the permanency of one-the Infusoria—as a distinct primary division. Nor, in view of the many analogies between the Mollusca and the Molluscoida, the Annulosa and the Annuloida, do I think it very improbable that, hereafter, some common and distinctive characters may possibly be discovered which shall unite these pairs respectively. But the discoveries which shall effect this simplification have not yet been made, and our classification should express not anticipations, but facts.

I have not thought it necessary or expedient, thus far, to enter into any criticism of the views of other naturalists, or to point out in what respect I have departed from my own earlier opinions. But Cuvier's system of classification has taken such deep root, and is so widely used, that I feel bound, in conclusion, to point out how far the present attempt to express in a condensed form the general results of comparative anatomy departs from that embodied in the opening pages of the "Régne Animal."

The departure is very nearly in the ratio of the progress of knowledge since Cuvier's time. The limits of the highest

group, and of the more highly organized classes of the lower divisions, with which he was so well acquainted, remain as he left them; while the lower groups, of which he knew least, and which he threw into one great heterogenous assemblage,-the Radiata,-have been altogether remodelled and rearranged. Milne-Edwards demonstrated the necessity of removing the Polyzoa from the radiate mob, and associating them with the lower Mollusks. Frey and Leuckart demonstrated the subregnal distinctness of the Calenterata. Von Siebold and his school separated the Protozoa, and others have completed the work of disintegration by erecting the Scolecida into a primary division, of Vermes, and making the Echinodermata into another. Whatever form the classification of the Animal Kingdom may eventually take, the Cuvierian Radiata is, in my judgment, effectually abolished: but the term is still so frequently used, that I have marked out those classes of which it consisted in the diagram of the Animal Kingdom (p. 6), so that you may not be at a loss to understand the sense in which it is employed.

LECTURE VI.

ON THE CLASSIFICATION OF ANIMALS.

THE SUBDIVISIONS OF THE MAMMALIA LARGER THAN

ORDERS.

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IN In my last lecture I endeavoured to point out the grounds upon which naturalists have arrived at the conclusion that the classes of the Animal Kingdom may be arranged together in larger groups or divisions, such as have been termed "provinces" and sub-kingdoms." If the time at my disposal for the consideration of Classification permitted me to do so, I should now, in the logical order of my discourse, take the opposite course; and turning again to the list of classes, I should endeavour to indicate in what manner they must be subdivided into sub-classes, orders, and lesser divisions. But it is needless to say that such a task as this would require many lectures, while I have only one to dispose of; and I propose to devote that one to a consideration of the classification of that class, which is in many respects the most interesting and the most important of any in the Animal Kingdom,-the class MAMMALIA.

A great many systems of classification of the Mammalia have been proposed, but, as any one may imagine from the nature of the case, only those which have been published within the last forty or fifty years, or since our knowledge of the anatomy of these animals has approached completeness, have now any scientific standing-ground. I do not propose to go into the history of those older systems, which laboured more or less under the disqualification of being based upon imperfect know

ledge, but I shall direct your attention at once to that important step towards dividing the Mammalia into large groups, which was taken by the eminent French anatomist, M. de Blainville, so far back as the year 1816. M. de Blainville pointed out that the Mammalia might be divided into three primary groups, according to the character of their reproductive organs, especially the reproductive organs of the female. He divided them into "Ornithodelphes," " Didelphes," "Monodelphes;" or, as we might term them, ORNITHODELPHIA, DIDELPHIA, MONODELPHIA. Now, I do not mean to assert that M. de Blainville defined these different groups in a manner altogether satisfactory, or strictly in accordance with all the subsequently discovered facts of science, but his great knowledge and acute intuition led him to perceive that the groups thus named were truly natural divisions of the Mammalia. And the enlargement of our knowledge by subsequent investigation seems to me, in the main, only to have confirmed De Blainville's views.

The division of the ORNITHODELPHIA comprises those two remarkable genera of Mammals, as isolated in geographical distribution as in structure,—Ornithorhynchus and Echidna,— which constitute the order Monotremata.

In these animals the angle of the lower jaw is not inflected, and the jaws are devoid of true teeth, one of the two genera only (Ornithorhynchus) possessing horny plates in the place of teeth. The coracoid bone extends from the scapula to the sternum, with which it is articulated, as in birds and most reptiles, and, as in many of the latter, there is an episternal bone. There is no marsupial pouch, though bones wrongly termed "marsupial” are connected with the pelvis. But it is to the structure of the female reproductive organs that the Ornithodelphia owe their name. The oviducts, enlarged below into uterine pouches, but opening separately from one another, as in oviparous vertebrates, debouch, not into a distinct vagina, but into a cloacal chamber, common to the urinary and genital products and to the fæces. The testes of the male are abdominal in position throughout life, and the vasa deferentia open into the cloaca, and not into a distinct urethral passage. The penis is indeed traversed by an

urethral canal, but it is open and interrupted at the root of that organ. In both sexes, the ureters pour the renal secretion, not into the bladder, which is connected with the upper extremity of the cloaca, but into the latter cavity itself.

In the brain, the corpus callosum is inconspicuous, though the question how far it can properly be said to be absent requires much more thorough investigation than it has yet received.* We are but very imperfectly acquainted with the reproductive processes of these animals, but it is asserted that the young are devoid of a placenta. The mammary gland has no nipple.

All

Like the Ornithodelphia, the division DIDELPHIA contains but a single order, the Marsupialia, the great majority of which, like the Ornithodelphia, inhabit Australia. They almost all have the angle of the lower jaw inflected, and all possess true teeth. The coracoid is, as in the higher Mammals, anchylosed with the scapula, and is not articulated with the sternum. have the so-called "marsupial" bones or cartilages—ossifications, or chondrifications, of the internal tendon of the external oblique muscle of the abdomen-and the females of almost all possess a fold of the skin of the abdomen above the pubis, constituting a "marsupium," or pouch, within which the young are nourished and protected in their early, helpless condition.

The oviducts open into vagina, which are more or less completely divided into two separate passages. The testes of the

*For a number of years I have entertained the gravest doubts respecting the accuracy of the doctrine put forth now nearly thirty years ago by Professor Owen, and almost universally received, that the corpus callosum is absent in Monotremes and Marsupials, and at one time I began to collect materials for the thorough investigation of the question; but other occupations intervened, and the plan was never carried out. Nevertheless, I have always expressed myself cautiously on this subject, and, as the text shows, I was particularly guarded when delivering the present lecture. At that time, in fact, I was well aware that my friend Mr. Flower had commenced a series of inquiries into the question, and such results as he had then obtained tended greatly to the increase of my scepticism. Mr. Flower has since been good enough to go carefully with me over the large series of drawings and preparations which he has made; and I am prepared to express my entire concurrence in his conclusion that the corpus callosum exists, distinctly developed, though not so well as in monodelphous, or placental, Mammals, in both the Didelphia and the Ornithodelphia.