The geographical distribution of animals, with a study of the relations ...

chap, in.] CONDITIONS AFFECTING DISTRIBUTION. 49adapted organisms. In the actual state of things, the physicalchanges that occur and have occurred through all geological epochsare larger and more varied. Almost every mile of land surfacehas been again and again depressed beneath the ocean ;most ofthe great mountain chains have either originated or greatlyincreased in height during the Tertiary period ; marvellousalterations of climate and vegetation have taken place over halfthe land-surface of the earth ; and all these vast changes haveinfluenced a globe so cut up by seas and oceans, by desertsand snow-clad mountains, that in many of its more isolatedland-masses ancient forms of life have been preserved, which,in the more extensive and more varied continents have longgiven way to higher types. How complex then must have beenthe actions and reactions such a state of things would bringabout ; and how impossible must it be for us to guess, in mostcases, at the exact nature of the forces that limit the range ofsome species and cause others to be rare or to become extinct !All that we can in general hope to do is, to trace out, more orless hypothetically, some of the larger changes in physicalgeography that have occurred during the ages immediately preceedingour own, and to estimate the effect they will probablyhave produced on animal distribution.We may then, by theaid of such knowledge as to past organic mutations as the geologicalrecord supplies us with, be able to determine the probablebirthplace and subsequent migrations of the more importantgenera and families; and thus obtain some conception of thatgrand series of co-ordinated changes in the earth and its inhabitants,whose final result is seen in the forms and the geographicaldistribution of existing animals.