226 ORIGIN OF LIFE IN AMERICA 



northern border of the United States, all over the States 

 and Central America. In South America its exact distribu- 

 tion is still unknown, but it possibly reaches Patagonia. The 

 cotton-tails and brush rabbits (Limnolagus) occur only in 

 the southern States. We thus note the remarkable fact 

 that rabbits of close relationship are separated in one 

 direction by a comparatively short intervening space of sea 

 water, in the other by a very much longer area of almost 

 uninterrupted land, which is tenanted almost exclusively by 

 the more distantly related hares. The other American 

 rabbits, the pigmy rabbit (Brachylagus), and the Popocatepetl 

 rabbit (Romerolagus), are confined to Mexico and western 

 North America. The south-western region must, therefore, 

 be looked upon as the centre of dispersal of the rabbits.* 



The fossil history of the American Leporidae is meagre in 

 the extreme. Only a few Oligocene species of the extinct 

 Palaeolagus are known, and these, according to Dr. Major, 

 seem to be ancestral to the modern genus Lepus. No fore- 

 runner of the existing Sylvilagus and Oryctolagus Jias yet been 

 found. It possibly lived in south-western North America 

 in early Tertiary times. Palaeolagus already possesses in- 

 cisors of the modern type, and Dr. Matthew thinks that we 

 may look among Eocene rodents, or even in the fauna of 

 Cretaceous deposits, for guidance as to the manner of evolu- 

 tion of the teeth of the Lagomorpha.f 



This leads us back once more to the general consideration 

 of the American Tertiary deposits and the affinities of their 

 fauna. These deposits, above all, ought to yield indications 

 as to whether there was a direct land connection between 

 south-western North America and western Europe across the 

 mid-Atlantic, such as the one I advocated. 



We are confronted in America by two grand problems, 

 says Professor Osborn,J one being the chronological correla- 

 tion of the purely fresh -water horizons with one another, the 

 other the chronological correlation of American horizons with 

 Eurasiatic vertebrate horizons. When these are worked out, 

 continues the same writer, we shall be able to establish a 



* Nelson, E. W., " Rabbits of North America." 



t Matthew, W. D., " A Horned Eodent from Colorado," p. 307. 



t Osborn, H. F., " Cenozoic Mammal Horizons," pp. 2930. 



