DISTRIBUTION. 409 



and reptiles of the same orders ; and the " regularity and 

 evenness of the dentition of the Anoploiherium as contrasting 

 with that of existing Artiodactyles." * 



The facts thus summed up do not show that higher forms 

 have not arisen in the course of geologic time, any more than 

 the facts commonly cited prove that higher forms have arisen ; 

 nor are they regarded by Professor Huxley as showing this. 

 Were those which have survived from palaeozoic and mesozoic 

 days down to our own day, the only types; and did the 

 modifications, rarely of more than generic value, which these 

 types have undergone, give no better evidences of increased 

 complexity than are actually given by them ; then it would be 

 inferable that there has been no appreciable advance. But 

 there now exist, and have existed during the more recent 

 geologic epochs, various types which are not known to have 

 existed in earlier epochs some of them widely unlike these 

 persistent types and some of them nearly allied to these 

 persistent types. As yet, we know nothing about the origins 

 of these new types. But it is possible that causes like those 

 which have produced generic differences in the persistent 

 types, have, in some or many eases, produced modifications 

 great enough to constitute ordinal differences. If structural 

 contrasts not exceeding certain moderate limits are held to 

 mark only generic distinctions; and if organisms displaying 

 larger contrasts are regarded as ordinally or typically distinct ; 

 it is obvious that the persistence of a given type through a 

 long geologic period without apparently undergoing devia- 

 tions of more than generic value, by no means disproves the 

 occurrence of far greater deviations in other cases; since 



* Since this passage was written, in 1863, there has come to light much 

 more striking evidence of change from a more generalized to a less general- 

 ized type during geologic time. In a lecture delivered by him in 1876, Prof. 

 Huxley gave an account of the successive modifications of skeletal structure 

 in animals allied to the horse. Beginning with the Orohippus of the Eocene 

 formation, which had four complete toes on the front limb and three toes on 

 the hind limb, he pointed out the successive steps by which in the Mesohippm, 

 Jfiohippus, Protohippus, and Pliohippus, there was a gradual approach to the 

 existing horse. 



