on its right side a well-marked, superior bridging convolution, coming for a considerable part of its length nearly or quite to a level with the lobes it connects. Tiedemann's figure of the chimpanzee's brain leads us, by its imperfectly-marked operculum, to the same conclusion as its sharply drawn one did in the case of the orang. The law of correlation of forms is a safe guide to us, when we have to predict what will be found in the lower organisms of well-marked families; it loses its in- flexibility, and becomes but a leaden rule, when we come to examine the most perfectly evolved species in such families. In the higher species of the order, apes, as in the higher varieties of the species, man, we find variability the rule, uniformity the exception; in the lower species, as in the lower varieties of man, the reverse condition obtains. The variability which we have seen to exist in the species chimpanzee, is no inconsiderable proof of its high relative rank in its own order.
But there is a second connecting bridge passing between the occipital and the parietal lobes. This convolution is invariably present, and invariably superficially placed in man; it is as invariably absent in both the anthropoid apes. In man it is always a large, easily recognizable structure; and in cases such as those which our fourth human brain may be taken to exemplify, or exaggerate, it will often be found to send a branch, as it were, in aid of the weakened superior bridge. The vacuity which in the apes corresponds to what is invariably a convolution of importance in man, may be seen in fig. 1, immediately posteriorly to 6; and in fig. 3, immediately below a. But this convolution, the "deuxième pli de passage" of Gratiolet, absent without exception in the Old World apes, and present equally invariably in man, is found also in two New World monkeys, the cebus capucinus[1] possessing it without, the ateles possessing it in company with its fellow.[2]
There is yet a third structure—"the Lobule of the Marginal Convolution"—to be treated of. In man it lies above the upper end of the fissure of Sylvius; and it may not unfairly be represented in our figure 1, by the convolution which lies immediately to the spectator's left of 5. Of it M. Gratiolet speaks in the following language:—"Cet lobule est particulier à l'homme et ne se trouve pas[3] ni dans l'orang ni dans le chimpanzee." But I find nowhere in M. Gratiolet's work any repetition of this striking statement: indeed it loses a good deal of its force, when we find the qualifying words "souvent assez grand" applied to this peculiarly anthropic lobule in the sentence immediately preceding the one we have quoted. And in the coloured diagrams, which speak so plainly, by their various hues, of the varied relations in extent and arrangement which may obtain among different brains, I find no separate colour assigned to this peculiarly separable lobe—no such distinction is awarded to them as there is to the bridging "plis de passage;" which, nevertheless, are not asserted to be exclu-
