GENERAL CONCLUSIONS. 791 



of existing ruminants, 1 that extinct species was seen to favour 

 rather than oppose the idea of organisation by secondary law. 



The discovery of the remains of the Hipparion 2 supplied one 

 of the links, required by Cuvier, between the Pal&otherium and 

 the Horse of the present day, and it is still more significant of 

 the fact of filiation of species that the remains of such three-toed 

 Horses are found only in deposits of that tertiary period which 

 intervene between the older palaeotherian one and the newer 

 strata in which the modern Horse first appears to have lost its 

 lateral hooflets. These relations I illustrated in my Lectures on 

 Fossil Mammalia at the School of Mines (1857) by the diagram, 

 fig. 614. 



Other evidences of gradation, in the case in question, have 

 been brought to light. The molar series of the Horse includes 

 six large complex grinders, individually recognisable by develop- 

 mental characters as they are symbolised in fig. 280, p. 352. The 

 representative of the first premolar is minute and soon shed. Its 

 homologue in Pal&otherium is functionally developed and re- 

 tained, the type-dentition being adhered to. 3 In Hipparion, d \ 

 is succeeded by a p \ 4 smaller than in JPal&otherium, but func- 

 tional, with inflected folds of enamel on the grinding surface, and 

 permanent. It exemplifies a condition intermediate to that in 

 Palceotherium and Equus. It is not that the jaws of the Horse 

 are too short to hold the full complement of grinders : on the 

 contrary they are relatively longer than in the Palasothere, being 

 specially produced between the grinders and cutters : the first 

 grinder might seem, indeed, to have been taken away in order to 

 add to the space for the application of the ' bit.' The transitory 

 and singularly small and simple denticle, fig. 614, p i, compared 

 with the large contiguous massive molar, m i, in the Horse, ex- 

 emplifies the rudiment of an ancestral structure, in the same 

 degree as does the hoofless ( splint-bones,' ib. Equus., n, iv. : just 

 as the spurious hoofs dangling therefrom in Hipparion, ib. 1 1. iv., 

 are retained rudiments of the functionally developed lateral hoofs 

 in the broader foot of Palcp other ium, ib. n. iv. 



Other missing links of this series of species have been supplied ; 

 as, e. g., by the Paloplotherium 5 of the newer eocene of Hordwell, 



1 CLXXX. p. 367. 



2 cccm". torn. ii. p. 25 (1832). Another species was discovered in the Miocene at 

 Eppelsheira the ' Hippotherium' of Kaup ; a third in deposits of similar age on the 

 Sewalik Hills ; a fourth, Hipparion prostylum, Grv., at Vaucluse, in the south-east of 

 France, in deposits ' peut-etre plus recents que la mollasse dans ces localites.' 

 cccxxx". p. 432. 



3 v. PI. 35, figs. 4, 5, 6. < cccii". PI. 19, figs. 1, 1 a. 



5 This modification, as the Palaotherium ovinum, Aymard, began to be shown, at 



