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Show *200 MR. R. LYDEKKER OX THE CALLOSITIES [Mar. 3, There 1 las, however, long existed an idea that the equine callosities are remnants of' a vanished toe. Somewhat analogous to this idea is a theory, which has been advanced in a paper lately read before the Royal Society of Edinburgh by Prof. Ewart1. That gentleman, it appears, has based his investigations to a very great extent on the condition obtaining in the foetus; and has arrived at the conclusion that the callosities in question have nothing to do with glands, but that they represent certain footpads of polydactyle mammals. According to Prof. Ewart's view, the fore-callosity of the Horse is homologous with the supplemental pad on the fore foot of the Dog, while the hind callosity (which is wanting in all existing Equiclce save the true Horse and the so-called Equus przewalskii) corresponds to the hinder1 plantar pad of the Banded Anteater (Myrmecobius fasciatus). Such correlations will, I venture to think, scarcely be taken seriously by the great majority of zoologists; and 1 shall therefore confine my attention to the question whether the identification of these callosities with foot-pads generally is well founded. In all cases my observations are confined to the condition obtaining in adult animals. It may be added that I purposely brought these observations to the notice of the Society before the publication of Prof. Ewart's paper, since I treat the subject from a different standpoint. In the first place, the callosities on both limbs of the Horse are situated on the inner surface, whereas, if they represented vestigial foot-pads, their position should be, primd facie, on the hinder aspect, as is the case with the ergot. It might, indeed, be argued that they have changed their original position, but of such a shifting there is no evidence in the adult. A second, and perhaps more important, objection to the foot-pad theory may be drawn from the fact that the callosities in the fore-limb are situated above the so-called knee-joint (carpus), and are therefore altogether higher up than any of the foot-pads of plantigrade mammals. Unless, therefore, another shift of position has taken place, the fore-callosities do not represent foot-pads. This argument, it may be mentioned, was used by Sir W. H. Flower to disprove the theory that the callosities are remnants of the lateral digits. The hind-callosities, on the contrary, are situated a short distance below the joint of the hock (tarsus), and are therefore on a part of the limb, albeit on its inner side, which is included in the foot of a plantigrade mammal. If, however, the front callosity be regarded as serially homologous in a general sense with the hind one-and this is an integral part of Prof. Ewart's theory,-it will be evident that in the event of the former not being a foot-pad, the same will hold good for1 the latter. A third, and perhaps stronger, objection may be urged against the foot-pad theory. On the assumption that the callosities of the existing Equidm are vestiges of foot-pads, it is clear that these 1 See ‘ Nature,' vol. lxvii. p. 239 (1903). |
