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a higher sphere of existence. In this destination alone may we discern an adequate end and purpose in the great organic scheme developed upon our planet.

The progressive gradations in this scheme will be further exemplified as I proceed to explain the principles and characters by which I have been guided in the formation of the primary groups or divisions of the class Mammalia.

Prior to the year 1836 it was held by comparative anatomists that the brain in Mammalia differed from that in all other vertebrate animals by the presence of the large mass of transverse white fibres, called 'corpus callosum' by the anthropotomist; which fibres, overarching the ventricles and diverging as they penetrate the substance of either hemisphere of the cerebrum, bring every convolution of the one into communication with those of the other hemisphere, whence the other name of this part—the 'great commissure.' In that year I discovered that the brain of the kangaroo, the wombat, and some other marsupial quadrupeds, wanted the 'great commissure;' and that the cerebral hemispheres were connected together, as in birds, only by the 'fornix' and 'anterior commissure1.' Soon afterward, I had the opportunity of determining that the same deficiency of structure prevailed in the Ornithorhynchus and Echidna*.

As many other modifications of structure, more or less akin to those characterizing birds and reptiles, were found to be associated with the above oviparous type of brain, together with some remarkable peculiarities in the economy of reproduction 3, I pointed out that the Mammalia might be divided into 'placental' and 'implacental4.'

Impressed, however, with the fact that such binary division, like that which might be based upon the leading differences of dentition, was too unequal to be natural, the larger

1 See Philosophical Trans, for 1837, p. 87.

3 Art. Monotbemata, Cyclopmdia of Anatomy and Physiology,Vo\. III. p. 38 3.

3 Art. Mabsupialia, torn. cit. p. 257.

4 Art. Mammalia, torn. cit. p. 244.

group never presenting the same degree of correspondence of organic structure as the smaller moiety, I continued to pursue investigations, with the view of gaining an insight into the more natural and equivalent primary groups of the Mammalia; having my attention more especially directed to the cerebral organ in this quest.

In 1842, I was able to demonstrate, in the 'Hunterian Course of Lectures' delivered at the Royal College of Surgeons, the leading modifications of the mammalian brain, and their peculiar value in classification by reason of their association with concurrent modifications of other systems of organs.

Nevertheless there were genera of Mammals, e. g. the sloths, anteaters, armadillos, roussettes, giraffes, rhinoceroses, &c. to which the cerebral test had to be applied. Fortunately the rare species of these genera successively arrived at the Zoological Gardens in London, and afforded me the means of applying that test; so that, at length, having dissected the brain in one species at least, of almost every genus or natural family of the Mammalian class, I felt myself in a position to submit to the judgment of my fellow-labourers in zoology, at the Linnaean Society, in 1857, the generalised results of such dissections, comprising a fourfold primary division of the Mammalia, based upon the four leading modifications of cerebral structure in that class.

In some mammals the cerebral hemispheres are but feebly and partially connected together by the 'fornix' and 'anterior commissure:' in the rest of the class the part called 'corpus callosum' is added, which completes the connecting or 'commissural' apparatus.

With the absence of this great superadded commissure1 is associated a remarkable modification of the mode of development of the offspring, which involves many other modifications; amongst which are the presence of the bones called 'marsupial,' and the non-development of the deciduous body

1 On the Structure of the Brain in Marsupial Animals, Philos. Trans. 1837.

concerned in the nourishment of the progeny before birth, called 'placenta ;1 the young in all this 'implacental' division being brought forth prematurely, as compared with the rest of the class.

This first and lowest primary group, or subclass, of Mammalia is termed, from its cerebral character, Lyencephala1, —signifying the comparatively loose or disconnected state of the cerebral hemispheres. The size of these hemispheres (fig. 4, A) is so small that they leave exposed the olfactory ganglions (a), the cerebellum (c), and more or less of the optic lobes (b); their surface is generally smooth; the anfractuosities, when present, are few and simple.

Fig- 5

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Brain of Beaver.

The next well marked stage in the development of the brain is where the corpus callosum (indicated in fig. 5, by the dotted lines d, d) is present, but connects cerebral hemispheres as little advanced in bulk or outward character as in the preceding subclass; the cerebrum (a) leaving both the olfactory lobes (a) and cerebellum (c) exposed, and being commonly

1 to loose; ^y/t^oXot, brain.

smooth, or with few and simple convolutions in a very small proportion, composed of the largest members, of the group. The mammals so characterized constitute the subclass LissenCephala1 (fig. 5).

The third leading modification of the Mammalian cerebrum is such an increase in its relative size, that it extends over more or less of the cerebellum; and generally more or less over the olfactory lobes. Save in very few exceptional cases of the smaller and inferior forms of Quadrumana (fig. 6), the superficies is folded into more or less numerous gyri or convolutions (fig. 7),—whence the name Gyrencephala, which I propose for the third subclass of Mammalia2.

Fig. 7.

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Brain of Chimpanzee.
(Half nat. size).

In Man the brain presents an ascensive step in development, higher and more strongly marked than that by which the preceding subclass was distinguished from the one below it. Not only do the cerebral hemispheres overlap the olfactory lobes and cerebellum, but they extend in advance of the

1 \ia<ros, smooth; {yn4<pa.\os, brain.

2 yvp6w, to wind about; fyic^aXos, brain.

one and further back than the other (figs. 8 & 9). Their posterior development is so marked that anthropotomists have assigned to that part the character and name of a'third lobe:' it is peculiar and common to the genus Homo: equally peculiar is the 'posterior horn of the lateral ventricle' and the 'hippocampus minor,' which characterize the hind lobe of each hemisphere. Fig 8The superficial grey matter of the cerebrum, through the number and depth of the convolutions, attains its maximum of extent in Man.

Peculiar mental powers are associated with this highest form of brain, and their consequences wonderfully illustrate the value of the cerebral character; according to my estimate of which, I am led to regard the genus Homo as not merely a representative of a distinct order, but of a distinct subclass,oftheMammalia, for which I propose the name of Archencephala (fig. 9)l. Ib. Side view, one-third nat. size.

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Brain of Negro, upper view.

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