PEIMITIVE CHARACTERS. 45 



This system, it will be observed, is almost exactly identical with that 

 employed in the preceding pages as the standard of comparison for the 

 Vertebrata. Yet it has resulted, from a most careful comparison of both 

 faunae and florae of America with this standard scale, that two distinct 

 paleontological series have to be adopted, the one for the vertebrate life 

 and the other for the plants, of the Western Continent. If this result be 

 accurate, and there appears to be no avoiding it, an explanation must be 

 sought. There are only two possible ones ; either the animal life of North 

 America has lagged behind that of Europe by one period during past geo- 

 logic time ; or, secondly, the vegetable life of America has been equally in 

 advance of that of Europe during the same period. In other words, if the 

 plant-life of the continents was contemporaneous, ancient types of animals 

 remained a period longer in North America than in Erope. If animal life 

 was contemporaneous, plant-life had advanced by one period in Europe 

 beyond that which it had attained in North America. In any case, either 

 the faunal or the floral standard of estimation of geologic age of strata for 

 North America is a false one, since there can be but one standard of com- 

 parison for anything. But this great fact being understood, the evidence of 

 each of the great departments of life possesses its own intrinsic value. 



In conclusion, it may be observed that the lacunae in the series as pre- 

 sented by one continent, render us dependent on the other for the evidence 

 necessary for the complete elucidation of the laws of the creation of animal 

 life. Phylogenies can be thus constructed which would otherwise be impos- 

 sible, and the results of researches into the earliest types of Vertebrata 

 become intelligible. Thus I have been able to prove, in support of a thesis 

 published in 1874, that the earliest Ungulate Mammalia were pentadactyle 

 and plantigrade. I have also shown that the ankle-joint had not, in the 

 primitive Mammalia, the hinge-like character that it has in the later ones, 

 but that it is without the interlocking superior articulation. The small size 

 of the brain of early Mammalia, already pointed out by Lartet, has received 

 extensive confirmation by the researches of Marsh, who has also shown the 

 progressive increase in size of the whole body in various mammalian lines. 

 To these results I have added another, which is derived from the study of 

 numerous Permian Vertebrata, viz, that the earliest land vertebrates had a 

 persistent chorda dorsalis. 



