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 



