Next in importance, perhaps, to the elevations and sinkings (traces of one or the other of which are more or less manifest in almost every region of the world), _natural barriers_ may be cited,--as presenting, not unfrequently, insurmountable obstacles to the self-dissemination of the insect tribes. By natural barriers, however, I would be understood to imply natural _primary_ barriers,--or, in other words, such as have continued as barriers ever since the present animals and plants came into existence upon the earth. For, the _ocean_ (by way of illustration) is a natural barrier; and yet it is not necessarily a primary one, as may be readily gathered from the above remarks, in which the results of _subsidences_ are discussed,--subsidences which have had the effect of letting it in over portions of an _already tenanted_, and unbroken, continent. Mountain-chains, also, are barriers; but it may happen that they have not been so from the beginning,--as in instances, for example, where they have been gradually upraised during periods geologically recent. But both sea and alpine ranges are barriers, when (as usually happens) they have remained as such since the creation of the several species which now inhabit our globe. Mr. Darwin has acknowledged this distinction, whilst commenting upon the marked divergence of the faunas on the eastern and western slopes of the Cordillera. "This fact," says he, "is in perfect accordance with the geological history of the Andes; for these mountains have existed as a great barrier since the present races of animals have appeared; and therefore, unless we suppose the same species to have been created in two different places, we ought not to expect any closer similarity between the organic beings on the opposite sides of the Andes, than on the opposite shores of the ocean. In both cases, we must leave out of the question those kinds which have been able to cross the barrier,--whether of solid rock or salt-water[69]."