CHAP. X.] 
THE RATE OF ORGANIC CHANGE. 
221 
been wanting. In the first place, every continent, though per- 
manent in a general sense, has been ever subject to innumerable 
physical and geographical modifications. At one time the total 
area has increased, and at another has diminished ; great plateaus 
have gradually risen up, and have been eaten out by denudation 
into mountain and valley ; volcanoes have burst forth, and, after 
accumulating vast masses of eruptive matter, have sunk down 
beneath the ocean, to be covered up with sedimentary rocks, and 
at a subsequent period again raised above the surface ; and the 
loci of all these grand revolutions of the earth’s surface have 
changed their position age after age, so that each portion of 
every continent has again and again been sunk under the ocean 
waves, formed the bed of some inland sea, or risen high into 
plateaus and mountain ranges. How great must have been the 
effects of such changes on every form of organic life ! and it is 
to such as these we may perhaps trace those great changes of 
the animal world which have seemed to revolutionise it, and 
have led us to class one geological period as the age of rep- 
tiles, another as the age of fishes, and a third as the age of 
mammals. 
But such changes as these must necessarily have led to re- 
peated unions and separations of the land masses of the globe, 
joining together continents which were before divided, and 
breaking up others into great islands or extensive archipelagoes. 
Such alterations of the means of transit would probably affect 
the organic world even more profoundly than the changes of 
area, of altitude, or of climate, since they afforded the means, at 
long intervals, of bringing the most diverse forms into competi- 
tion, and of spreading all the great animal and vegetable types 
widely over the globe. But the isolation of considerable masses 
of land for long periods also afforded the means of preservation 
to many of the lower types, which thus had time to become 
modified into a variety of distinct forms, some of which became 
so well adapted to special modes of life that they have continued 
to exist to the present day, thus affording us examples of the 
life of early ages which would probably long since have become 
extinct had they been always subject to the competition of the 
more highly organised animals. As examples of such excessively 
