chap, vi.] GEOGRAPHICAL AND GEOLOGICAL CHANGES. 
99 
General Stability of Continents with 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 would 
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 
