‘CHAP. 11.] 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, which, 
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. 
