CHAP. VI.] GEOGRAPHICAL AND GEOLOGICAL CHANGES. 99 



General Stability of Continents ivith Constant Change of Form. 

 — It will be observed that the very same evidence which has been 

 adduced to prove the general stability and permanence of our 

 continental areas also goes to prove that they have been sub- 

 jected to wonderful and repeated changes in detail. Every 

 square mile of their surface has been again and again under 

 water, sometimes a few hundred feet deep, sometimes perhaps 

 several thousands. Lakes and inland seas have been formed, 

 have been filled up with sediment, and been subsequently raised 

 into hills or even mountains. Arms of the sea have existed 

 crossing the continents in various directions, and thus completely 

 isolating the divided portions for varying intervals. Seas have 

 been changed into deserts and deserts into seas. Volcanoes 

 have grown into mountains, have been degraded and sunk 

 beneath the ocean, have been covered with sedimentary 

 deposits, and again raised up into mountain ranges ; while 

 other mountains have been formed by the upraised coral reefs 

 of inland seas. The mountains of one period have disappeared 

 by denudation or subsidence, while the mountains of the suc- 

 ceeding period have been rising from beneath the waves. The 

 valleys, the ravines, and the mountain peaks, have been carved 

 out and filled up again ; and all the vegetable forms which 

 clothe the earth and furnish food for the various classes of 

 animals have been completely changed again and again. 



Effect of Continental Changes on the Distribution of Animals. — 

 It is impossible to exaggerate, or even adequately to conceive, 

 the effect of these endless mutations on the animal world. 

 Slowly but surely the whole population of living things must 

 have been driven backward and forward from east to west, or 

 from north to south, from one side of a continent or a hemi- 

 sphere to the other. Owing to the remarkable continuity of 

 all the land masses, animals and plants must have often been 

 compelled to migrate into other continents, where in the 

 struggle for existence under new conditions many w^ould 

 succumb; while such as were able to survive would consti- 

 tute those wide-spread groups whose distribution often puzzles 

 us. Owing to the repeated isolation of portions of continents 

 for long periods, special forms of life would have time to be 



