104 GEOGRAPHICAL DISTRIBUTION 
the Dasypodide or Armadillos, of which one species has crept 
northward as far as Texas. Almost equally characteristic are the 
numerous Opossums, the majority of which belong to the genus 
Didelphys. Finally, it should be observed that the West Indies are 
distinguished from the rest of the region by the absence of Primates, 
Carnivora, and Edentates. 
Aquatic Mammals.—Many mammals grouped for the present 
. purpose as terrestrial pass a great portion of their lives in brooks, 
lakes, or rivers, and, being dependent upon such waters for ob- 
taining their subsistence, are necessarily confined to their vicinity ; | 
but the truly aquatic mammals, or those living constantly in the 
water, and unable to move their quarters from place to place by 
land, are the orders Cetacea and Sirenia, with which may also be 
grouped the Seals, forming the Pinniped division of the order 
Carnivora. 
For the marine Cetacea, animals mostly of large size and 
endowed with powers of rapid locomotion, there are obviously no 
barriers to universal distribution over the surface of the earth 
covered by sea, except such as are interposed by uncongenial 
temperature or absence of suitable food. Nevertheless it was 
thought some years ago that the fact of a Whale or a Dolphin 
occurring in a sea distant from that in which it had usually been 
found was sufficient justification for considering it as a distinct 
species and imposing a new name upon it. There are now, 
however, so many cases known in which Cetaceans from the 
northern and southern seas, from the Atlantic and Pacific Oceans, 
present absolutely no distinguishing external or anatomical charac- 
ters upon which specific determination can be based that the 
opposite view is gaining ground; and, since some species are un- 
doubtedly very widely distributed, being in fact almost cosmopolitan, 
there seems little reason why many others should not be included in 
the same category. The evidence is satisfactory enough in those 
instances in which the intermediate regions are inhabited by the same 
forms ;—the cases of “ continuous areas ” of distribution. In those in 
which the areas of distribution are apparently discontinuous, there 
may be more room for doubt; but it must not be forgotten that the 
negative evidence is here of much less value than in the case of 
land animals, since the existence of Cetaceans in any particular part 
of the ocean may be easily overlooked. The great Sperm Whale 
(Physeter macrocephalus) is known to be almost cosmopolitan, in- 
habiting or passing through all the tropical and temperate seas, 
although not found near either pole. At least three of the well- 
known species of Rorqual (Balenoptera) of the British coasts are 
represented in the North Pacific, on the South American shores, 
and near New Zealand, by species so closely allied that it is difficult 
to point out any valid distinctive characters, though it may perhaps 
