xi THE EVEN-TOED UNGULATES 151 



visible, while in the Cervidse of the more recent tertiary 

 periods, and especially the pleistocene and living species, 

 these same cavities are so deep that, whatever be the state of 

 dentition, the bottom cannot be seen. This (he says) is a 

 perfectly reliable rule for distinguishing the ancient from the 

 more modern forms of deer, and can be applied to other 

 animals as well as the Cervidas. From it he surmises that 

 the duration of the life of modern is greater than that of 

 ancient deer. The same careful observer also remarks that in 

 many natural groups a gradual progress is observed in the 

 volume of the brain and complexity of its surface, as deduced 

 from casts of the interior of the skull, from which fact he 

 concludes that a gradual growth of vital energy and intelli- 

 gence has occurred as the effect of the tendency towards 

 improvement, of which the cause is always acting, and the 

 limits indefinite. 



Thus the history of the Even-toed Ungulates tells the 

 same story as that of the Perissodactyles. The modern forms 

 are placed along lines which converge towards a common 

 centre. Moreover, the lines of both groups, to a certain extent, 

 approximate ; but within the limits of our knowledge they 

 do not meet. Both artiodactyles and perissodactyles existed 

 low down in the eocene, just as Carnivores, Insectivores, bats, 

 rodents, and other great groups then existed with boundary 

 lines as distinctly marked as now. 



Was the order,i according to which the introduction of 

 new forms seems to have taken place since that epoch, then 

 entirely changed ? or did it continue as far back as the period 

 when these lines would have been gradually fused into a 

 common centre ? 



Here we are landed in the region of pure speculation ; but 

 bolder travellers than I have endeavoured to penetrate its 

 mysteries, as may be seen by a perusal of Professor Huxley's 

 presidential address to the Geological Society for 1870. 



I have so far confined myself within the region of the 

 known, and shown that at least in one group of animals the 

 facts which we have as yet acquired point to the former 

 existence of intermediate forms, so numerous that they go far 

 to discredit the view of the sudden introduction of new species. 



