228 ISLAND LIFE 



the distribution of animals. Slow elevations of the land 

 would produce another set of changes, by affording an ex- 

 tended area in which the more dominant species might in- 

 crease their numbers ; and . by a greater range and variety 

 of alpine climates and mountain stations, affording room 

 for the development of new forms of life. 



Geograi^liical 3futatio7is as a Motive Poiver in Bringing 

 about Organic Changes. — Now, if we consider the various 

 geographical changes which, as we have seen, there is 

 good reason to believe have ever been going on in the 

 world, we shall find that the motive power to initiate and 

 urge on organic changes has never been wanting. In the 

 first place, every continent, though permanent 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 reptiles, another 

 as the age of fishes, and a third as the age of mammals. 



But such changes as these must necessarily have led to 

 repeated 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 aflect the organic world even 

 more profoundly than the changes of area, of altitude, or 



