MUSEUM OF COMPARATIVE ZOOLOGY. 249 



the other. It hence seems unnecessary to suppose the former existence 

 of an Atlantic continent to explain their present distribution. It is also 

 a noteworthy fact that no cases of close affinity among the mammals 

 inhabiting these two continents occur in species that do not range very 

 far to the northward, as in the Felidce r for example, where the only case 

 at all suggestive of identity, or even of close relationship, occurs be- 

 tween the Lynx canadensis of Northern North America and the Lynx 

 lynx of Northern Europe ; both of which species range the farthest 

 north of any of their family, and reach the Arctic regions. 



All the circumpolar species, the beaver alone excepted, pertain to 

 the most highly organized groups found in the colder portion of the 

 northern hemisphere, and to which belong not only all the widely rang- 

 ing species of the north temperate and boreal regions, but those of 

 this character everywhere. With three exceptions, all are carnivores. 

 Two of the others are ruminants, and one is a rodent. 



The species most highly organized in their respective families, orders, 

 or classes are almost universally those that possess the widest geo- 

 graphical distribution ; partial exceptions occur only in groups where 

 the means of locomotion is specialized, or unusually developed, as in the 

 bats among mammals. The shrews, moles, and rodents, which comprise 

 about three fifths of the species of the North American mammals, are 

 groups of low structural rank, and abound in species of comparatively 

 local distribution. In this great number there are but five dt six, 

 allowing the broadest latitude in respect to the limitation of the species, 

 that at all approach to a continental distribution, and only three as the 

 species are usually restricted.* This is about two one-hundredths of 

 one per cent. Only one can be regarded as identical with any 

 Old World species. In the canivores, on the other hand, excluding 

 sub-tropical and nominal species, the number of those that range 

 over most of the continent reaches nearly seventy-five per cent, while 

 fifty per cent, or one half, are identical with Old World species. In 

 the ruminants, which rank below the carnivores, but far above the 

 rodents and insectivores, the species having a similarly wide range on 

 this continent, number not far from thirty per cent. Several of them are 

 identical with Old World species. The bats, though a low group, are, 



* Castor fiber, Fiber zibethicus, and Jaculus hudsonins. Probably the following may be 

 added to the list of those that range across the continent: Erethizon dorsatus, Scvdrus 

 hudsonius, Pleromys volucella, Hesperomys leucopus. 



