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Show 650 PROF. HUXLEY ON THE [D three processes by which the Eohippus form has passed into Ecpxus as the expression of a threefold law of evolution. It is of profound interest to remark that this law, or generalized statement of the nature of the ancestral evolution of the Horses, is precisely the same as that which formulates the process of individual development in animals generally, from the period at which the broad characters of the group to which an animal belongs are discernible, onwards. After a mammalian embryo, for example, has taken on its general mammalian characters, its further progress towards its specific form is effected by the excessive growth of one part in relation to another, by the arrest of growth or the suppression of parts already formed, and by the coalescence of parts primarily distinct. This coincidence of the laws of ancestral and individual development creates a strong confidence in the general validity of the former ; and a belief that we may safely employ it in reasoning deductively from the known to the unknown. The astronomer who has determined three places of a new planet, calculates its place at any epoch however remote ; and if the law of evolution is to be depended upon, the zoologist who knows a certain length of the course of that evolution in any given case, may with equal justice reason backwards to the earlier but unknown stages. Applying this method to the case of the Horse, I do not see that there is any reason to doubt that the Eocene Equidce were preceded by Mesozoic forms which differed from Eohippus in the same way as Eohippus differs from Equus. And thus we are necessitated to conceive of a first term of the Equine series, which, if the law is of general validity, must needs have been provided with five subequal digits on each plantigrade foot, with complete, subequal antebrachial aud crural bones, with clavicles, and with, at fewest, 44 teeth, the cheekteeth having short crowns and simple-ridged or tuberculated patterns. Moreover, since Lartet's and Marsh's investigations have shown that the older forms of any given mammalian group have less developed cerebral hemispheres than the later, there is a prima facie probability that this primordial Hippoid had a low form of brain. Further, since the existing Horse has a diffuse allantoic placentation, the primary form could not have presented a higher, and may have possessed a lower, condition of the various modes by which the foetus derives nourishment from the parent among vertebrated animals. Such an animal as this, however, would find no place in any of our systems of classification of the Mammalia. It would come nearest to the Lemuroidea and the Insectivora, though the non-prehensile pes would separate it from the former, and the placentation from the latter group. A natural classification is one which associates together all those forms which are closely allied and separates them from the rest. But, whether in the ordinary sense of the word "alliance," or in its purely morphological sense, it is impossible to imagine groups of animals more closely allied than the primordial Hippoids are with then-descendants. Yet, according to existing arrangements, the ancestors |