388 CAUSES OF CL1MATAL CHANGE 



We are thus obliged to fall back upon the old Lyellian theory 

 of geographical changes, \vith such modifications as recent dis- 

 coveries have rendered necessary. Taking this as our guide, 

 we reach at once the important conclusion that the movements 

 and distribution of animals and plants, however dependent on 

 climate, altitude and depth, have, when regarded in connection 

 .with geological time, been primarily determined by those great 

 movements of the crust of the earth which have established 

 our islands, continents and ocean depths. These geographical 

 changes have also in connection with animal and vegetable 

 growth, deposition of sediments and volcanic ejections, fixed 

 even the stations, soils and exposures of plants and animals. 

 Thus, subject to those great astronomical laws which regulate 

 the temperature of our planet as a whole, our attention may be 

 restricted to the factors of physical geography itself. We 

 must, however, carry with us the idea that though the great 

 continents and the ocean depths may have been fixed through- 

 out geological time, their relative elevations, and consequently 

 their limits, have varied to a great extent, and are constantly 

 changing. 



We must also remember that something more than mere 

 cold is necessary to produce a glacial period. It has sometimes 

 been assumed that the tendency of an exceptionally cold winter 

 would necessarily be to accumulate so great a quantity of snow 

 and ice, that these could not be removed in the short though 

 warm summer, and so would go on accumulating from year to 

 year. Actual experience and observation do not confirm this 

 supposition. In those parts of North America which have a 

 long and severe winter, the amount of snow deposited is not in 

 proportion to the lowness of the temperature, but, on the con- 

 trary, the greatest precipitation of snow takes place near the 

 southern margin of a cold area, and the snow disappears with 

 great rapidity when the spring warmth sets in. Nor is there, as 

 has been imagined, any tendency to the production of fogs and 



