272 MAMMALS. 



Lesson, who grouped all the Pouclied Rats together, was, therefore, not so unreasonable a ijroposition 

 as it looks like. Were it not for the other anatomical differences I should follow his example. 



As to their geographical distribution, the whole of the externally-pouched species belong to 

 the New World ; the whole of the internally-pouched species to the Old World. The New-world 

 species are all North American, at least none reach further south than Central America. They 

 form two very distinct and strongly-marked groups ; so much so, that one of the objections taken 

 by Brandt in his revision of the order to Waterhouse's family of the Saccomyina, was the want of 

 affinity of these two groujDs. The one group, consisting of the genera GeO]MYS and Thomojiys, being 

 heavy, thickset, burrowing animals, in aj)pearance something between a Mole and a miniature 

 Beaver ; the other, Dipodomys and Perognathus, light, elegant, graceful, jumping creatures, with 

 the long hind-legs and short fore-legs of the Jerboa. 



The latter form is confined to the west side of the continent, the limiting boundary being, not 

 the Rocky Mountains, but the Missom-i district, where formei-lj' rolled the tertiary sea, stretching 

 almost from the Gulf of Mexico to the Arctic Sea (see Map 81). 



The former, our friend the " Sand- Rat of Canada," known throughout all the west of North 

 America as tlie Gopher,* again naturally breaks into two sections ; one the Geojiys, with the 

 superior incisors broad and grooved ; the other, the Tiiomomys, with these incisors narrow and 

 smooth. These are the pest and dread of the horticulturist wherever they occur. In some parts of 

 California, where they abound, nothing under-ground escapes them — turuijjs, carrots, and every 

 tuberous or bulbous plant is eaten up. Here, again, we have, even more markedly, the same 

 phenomenon of distribution which occurs with Dipodomys. The species of Geomys are confined to 

 the east of the continent, and those of Thomomys to the west. And what is the line of demarcation ? 

 the same as before, the tertiarj^ beds dividing the continent from the north to the south. There 

 are, however, two other specialties to be noted ; one on the north and one to the south. On 

 the north, Thomomys crosses the border line, and sends a species on to the shores of Hudson's 

 Bay. How comes this western species to have passed the line which seems to have been an 

 effectual barrier further to the south ? Probably by migration subsequent to the elevation of the 

 tertiary basin ; we have seen that migration is more likely to occvir with northern than soutliern 

 species, and we know of no break in the tertiary beds to the north of Nebraska ; still the whole 

 of that portion has not been thoroughly surveyed, and breaks may exist wliicli we do not yet 

 know of, by which the animal may have passed. 



The other point to be noted is, that to the south the eastern Geomys crosses the line of the 

 limiting boundary in Texas, and goes south into Mexico and Central America. The western 

 Dipodomys also goes south through Mexico into Central America, but that has no special significance 

 on this point. It is merely a continuation further south of its natural range. But it is different 

 with Geomys, which comes from the other side of the continent. If the reader will refer to Map 

 3 he will see, that there probably did exist a dry-land passage across the tertiary basin in the 

 north of Texas, exactly in the line by which this species has extended its range. 



There is another point on which the extent of the southern range of these species has a bearing ; 

 and that is, the limiting line between the faunas of North and of South America. These northern 

 species are found as far south as Guatemala. There are other facts of the same nature, but there 



* In the south-eastern states of North America that name is applied to the large Tortoise, and the Geomyna 

 are there called Salamanders. 



