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 has 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 wo 
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 Rodent from Colorado,” p. 307. 
t Osborn, H. F., “ Cenozoic Mammal Horizons,” pp. 29—30. 
