220 



ISLAND LIFE. 



[part t. 



Geographical changes would be still more important, and it 

 is almost impossible to exaggerate the modifications of the 

 organic world that might result from them. A subsidence of 

 land separating a large island from a continent would affect the 

 animals and plants in a variety of ways. It would at once 

 modify the climate, and so produce a series of changes from this 

 cause alone ; but more important would be its effect by isolating 

 small groups of individuals of many species and thus altering 

 their relations to the rest of the .organic world. Many of these 

 would at once be exterminated, while others, being relieved from 

 competition, might flourish and become modified into new 

 species. Even more striking would be the effects when two 

 continents, or any two land areas which had been long separated, 

 were united by an upheaval of the strait which divided them. 

 Numbers of animals would now be brought into competition 

 for the first time. New enemies and new competitors would 

 appear in every part of the country ; and a struggle would 

 commence which, after many fluctuations, would certainly result 

 in the extinction of some species, the modification of others, 

 and a considerable alteration in the proportionate numbers and 

 the geographical distribution of almost all. 



Any other changes which led to the intermingling of species 

 whose ranges were usually separate would produce corresponding 

 results. Thus, increased severity of winter or summer tempera- 

 ture, causing southward migrations and the crowding together 

 of the productions of distinct regions, must inevitably produce 

 a struggle for existence, which would lead to many changes both 

 in the characters and the distribution of animals. Slow eleva- 

 tions of the land would produce another set of changes, by 

 affording an extended area in which the more dominant species 

 might increase 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. 



Geographical Mutations as a Motive Power in hringing 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 



