chap, in.] CONDITIONS AFFECTING DISTRIBUTION. 49 



adapted organisms. In the actual state of things, the physical 

 changes that occur and have occurred through all geological epochs 

 are larger and more varied. Almost every mile of land surface 

 has been again and again depressed beneath the ocean ; most of 

 the great mountain chains have either originated or greatly 

 increased in height during the Tertiary period ; marvellous 

 alterations of climate and vegetation have taken place over half 

 the land-surface of the earth ; and all these vast changes have 

 influenced a globe so cut up by seas and oceans, by deserts 

 and snow-clad mountains, that in many of its more isolated 

 land-masses ancient forms of life have been preserved, winch, 

 in the more extensive and more varied continents have long 

 given way to higher types. How complex then must have been 

 the actions and reactions such a state of things would bring 

 about ; and how impossible must it be for us to guess, in most 

 cases, at the exact nature of the forces that limit the range of 

 some 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 or 

 less hypothetically, some of the larger changes in physical 

 geography that have occurred during the ages immediately pre- 

 ceeding our own, and to estimate the effect they will probably 

 have produced on animal distribution. We may then, by the 

 aid of such knowledge as to past organic mutations as the geo- 

 logical record supplies us with, be able to determine the probable 

 birthplace and subsequent migrations of the more important 

 genera and families ; and thus obtain some conception of that 

 grand series of co-ordinated changes in the earth and its in- 

 habitants, whose final result is seen in the forms and the geo- 

 graphical distribution of existing animals. 



