PROF. HUXLEY ON THE [DeC. 14, 



three processes by which the Eohippus form has passed into Equus 

 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 dis- 

 cernible, 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 suppres- 

 sion of parts already formed, and by the coalescence of parts pri- 

 marily distinct. 



This coincidence of the laws of ancestral and individual develop- 

 ment 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 deter- 

 mined 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 evo- 

 lution 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 EoJdppus \\\ the same way 

 as Eohijjpus 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 subequnl 

 digits on each plantigrade foot, with complete, subequal antebrachial 

 and crural bones, with clavicles, and witl), at fewest, 44 teeth, the cheek- 

 teeth having short c\'owns 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 nou-prehcnsile 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 their 

 descendants. Yet, according to existing arrangements, the ancestors 



