« AnteriorContinuar »
Such, for instance, are Rangifer (the Reindeer), Alces (the Elk), Haploceros (the mountain goat), Gulo (the glutton), and many others.
The following table, which gives a summary of the genera of this Sub-region, differs only from the summary of the Canadian Sub-region in the omission of the heading "Arctic and Palæarctic," since practically all the genera common to this Sub-region and the Palearctic Region are also found in the Canadian Sub-region.
SECTION VI.-THE EASTERN OR HUMID SUB-REGION
So far as peculiar forms go, the Eastern Sub-region is quite the least peculiar of the three. It contains only one genus strictly confined within its limits; this is Neofiber, with a single species commonly known as the "Round-tailed Musk-rat." This Rodent is found only in Florida, and is much less completely aquatic in its habits than the true Musk-rat (Fiber), which is spread over the rest of North America.
Taking the orders seriatim, the Marsupials are represented by the widely spread Virginian Opossum ; but the Edentates do not reach the Region at all. Of the Ungulates
only the genus Cariacus (the Virginian Deer) occurs, the Bison (though formerly inhabiting this Sub-region) not having been seen east of the Mississippi for the last forty or fifty years.
The Rodents, as in the other Sub-regions, make up the great mass of the mammalian genera, numbering seventeen in all, including Neofiber.
The Carnivores, Insectivores, and Bats do not differ very markedly from those of the Western Sub-region.
On the whole the Eastern Sub-region is not a very well-marked division; it differs from the Canadian chiefly in the non-existence of the numerous northern Palæogean types found there, and from the Western Sub-region in the absence of a good many characteristic desert-haunting forms, and also of several of the South American genera, which have spread up northwards from the Nearctic Region into the Western Sub-region, but which have not reached the more distant Eastern.
The following table gives a summary of the genera of this Sub-region, from which it will be seen that the total number (forty) of genera is markedly less than the corresponding number in the Arid Sub-region.
SECTION VII.-THE PAST HISTORY OF THE NEARCTIC MAMMAL-FAUNA
During the last twenty years the wonderful discoveries of American paleontologists have thrown a flood of light, not only on the past history of the Nearctic Region, but also on the evolution of many of the mammalian groups themselves. It is, therefore, very necessary, when reviewing the geographical distribution of the present mammalian fauna, to shortly recapitulate the more important results and conclusions arrived at from their writings.
A very useful and comprehensive summary of this work will be found in a paper by Professor Zittel (3), lately published in the Geological Magazine. The beds which contain the remarkably perfect remains above alluded to are found only in the western part of North America. Here, apparently, there existed throughout the Tertiary Epoch a series of great fresh-water lakes, on the sides and the bottoms of which were formed an almost continuous series of deposits with the remains of the animals of the surrounding districts embedded in them. The great interest of these discoveries lies in the fact that we can here trace the gradual formation and evolution of several of the mammalian orders as they at present exist. In the oldest beds the mammals resemble one another so closely that it is often impossible to assign them very definitely to any of the existing orders, although the germs of the commencing distinctive characters can even here be
In the later beds the various groups gradually differentiate themselves, until in the most modern of
the deposits the genera can all be definitely assigned to existing orders.
The earliest mammals that have been found in North America come from the Trias of North Carolina, but neither these nor any of the other mammalian remains of the Secondary Period tend to assist the geographical problems involved, or are of importance in the present juncture.
With the oldest Tertiary beds an entirely new fauna appears, and furnishes us with remains of forms belonging to various orders of which no traces can be found in the earlier Secondary deposits. The following is a short list of these deposits, together with their European equivalents, so far as they can be ascertained :—
In the lowest Eocene beds, not only the Genera, but even the Orders of mammals are in almost every case different from those at present existing. The greater number of these belong to two orders-Creodontia and Condylarthra; the former the progenitors of the modern Carnivora, the latter of the existing Ungulata. These
two, and indeed the other orders to which the mammals of this fauna have been assigned, all show considerable points of resemblance to one another, first in the possession of five toes on both limbs, which are provided with neither claws nor hoofs, but with a structure somewhat intermediate between the two, and, secondly, in their extremely small cerebral cavity. A similar, though much more incomplete Fauna has been found in certain beds of a corresponding age in Europe, the Genera of their fossil mammals being in most cases identical with those of the Nearctic Region.
In the next stage, the Wasatch beds, which correspond in age nearly to the London clay of England, a further development of the same fauna is found, with, however, the commencement of certain of the modern Orders; such, for instance, as the Perissodactyla (or Odd-toed Ungulates), the Rodents, the Insectivores, and the Lemurs. Here, too, so far as the scanty remains found in Europe allow us to form a comparison, there is a close similarity between the faunas of the two Regions.
In the succeeding "Bridger beds" of Mid-Eocene age is found the earliest evidence of the still surviving genus Didelphys (the Opossum). Here also marine Mammals. and Bats appear for the first time. But comparison of these remains with European forms is even more difficult than in the last case, owing to the scarcity of such fossils in beds of the same age in Europe.
In the Uintah beds of the Upper Eocene we first begin to find very distinct traces of differentiation between the European and the North American faunas, although a good many of the Genera mét with are still common to the two Regions.