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Show 96 ON CLASSIFICATION. 0 · his paper "On tho Characters, Principles of Divi ion, wen, In 1' " bl ' l l . and P n·m ary Gr·oups of the cia s Mamma Ia., pu IS 1oc 111 tho J , 1 of the Linnrean Soci ty ; thouo·h. rt should be add 'd ourna dd' . 1 that Professor Owen made certain a. 1t10ns to t 10 ~uclous furnished by Mr. Waterhouse, wlu ·h aro unquestiOnably original. . . Thus the "Lissencephala " of Pro£ s or Owen IS simply a now name for the group of Ma1nmals (" in ·which the cerebrum i.· destitute of convolutions, or nearly so ") indicated by Mr. Waterhouse ; and " Gyrencephala " is a lik v rhal equivalent for Mr. 'Vaterhouse's group of l\Imnmals characteri. eel by having the brain provided wi~h distinct convol~ti01~~· But Mr. Waterhouse does not mentron l\f an at all, wlnle I rofcssor Owen creates a new sub-cla. , Arr·chencephala, for the gonuR Homo, and substitutes the na1n "Lyencephala '' for In1placentalia, formerly applied to the Ornithodelphia and Dide~Jhia. In attempting to decide between the various classifications thus presented to us, the anon by which our judgment must be guided are simple enough. It is obvion,, in th first place, that the definition of a group, wh th r that d finition bo based on cerebral or on plac ntal charact rs, mu t be true, as a matter of fact, if any valu is to be attach d to th clas ·ification of which that definition fonn a 1 art. And, in the second place, it is clear tlmt tb definition of uach group must be distin tiv , that is to ay, it must not include the members of other groups. Applying these ond canon to the cla ification last mentioned, it appears to me to collap e at once. The sub-cla s Lissencephala, for xampl , is thus defined:-. "The corpus callosum i pr s nt, but connects cerebral hemispheres as little advan, d in bulk or outward haracter as in the preceding sub-cla ; th ereLrum leaving both tho olfactory lobe and corebellun1 xposed, and b ing ommonly sn1ooth: or with few and simpl convolut ions in a very small proportiOn, composed of the largest member of the gronp. The Mammals so characterised constitute the sub- lass L issencephala." L. c., p. 14. THE ARCHENCEPHALA. 97 On the other hand, the sub-class Gyrencephala receives the following definition:- ,,The third leading modification of the 1\Iammalian 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 tho smaller and inferior forms of the Quadrumana, the superficies are folded into more or less numerous gyri, or convolutions, whence the name Gyrencephala, which I propose for the third sub-class of Mammalia."-L. c., p. 18. I am quite unable to see what these so-called definitions define. If, for exa1nple, we place the brains of an Ant-eater, or of a Capybara, side by side with that of a Genett-the two former being Lissencephala, the latter one of the Gyrencephala-either ''definition" will apply equally well to either of the thre brains. All three have slightly convoluted brains; in all three the olfactory lobes and cerebellum are more or less uncovered; and nothing in the definitions of the sub-classes of this "eerebral classification" would enable an anatomist to say that any one of these three brains belonged to one sub-class rather than another. Since Mr. Waterhouse pointed out the fact, no one has doubted that, as a general rule, the brains of the so-called" Gyrencephala" are more eon voluted, size for size, than those of the "Lissencephala ;'' and the relations of the size and the zooloO'ical . . 5 positiOn of an animal to the characters of its cerebral surface have long since been well discussed by Gratiolet, Dareste, and others. But it is exactly because the rule is only a g neral one, and has many exceptions, that the degree of cerebral convolution must be rejected as the basis of the definition of any large group of Mammals. Thus far, we meet, in Professor Owen's definitions, with a certain foundation in fact, though it may not be such as is fitted ~o ~fford ground for classification: but the group "A rchencephala" rs rn a more unfortunate position. Our first canon comes into ?peration, and we must reject it, because the statements respectIng matters of fact in its definition are untrue. The words stand thus:- H |