The relation of the and at their point of junction is as yet a problem not fully solved. Where there is placed between the two an obstacle to migration of land and fresh water animals, as between the and the islands of the , the differential character of the faunæ are well marked; but where the northern continent gradually passes into the long isthmus of Mexico, the case is far different. Here no barrier obstructs the passage of animals from one region to the other and nothing but and causes can limit the range of the types of either fauna. A careful investigation of this subject must have an important bearing on the whole subject of the influence of on animal life. In general the studies into the zoölogy of Mexico have shown conclusively that the high plateau is populated by an animal life which differs from that of the eastward s, or . The highest land is known as the Tierra Alpina, and the intermediate region is the . As residents of the land incapable of performing migrations, the ' and ' offer the most excellent field for the determination of the boundaries of these districts.
— EDWARD DRINKER COPE