56 



ISLAND LIFE 



PAIIT I 



of animals lias been much neglected. We therefore devote 

 the present chapter to its consideration. 



As we have already shown in our first chapter that the 

 distribution of species, of genera, and of families, present 

 almost exactly the same general phenomena in varying 

 degrees of complexity, and that almost all the interesting 

 problems we have to deal with depend upon the mode of 

 dispersal of one or other of these ; and as, further, our 

 knowledge of most of these groups, in the higher animals 

 at least, is confined to the tertiary period of geology, it is 

 therefore unnecessary for us to enter into any questions 

 involving the origin of more comprehensive groups, such 

 as classes or orders. This enables us to avoid most of the 

 disputed questions as to the development of animals, and 

 to confine ourselves to those general principles regulating 

 the origin and development of species and genera which 

 were first laid down by Mr. Darwin thirty years ago, and 

 have now come to be adopted by naturalists as established 

 propositions in the theory of evolution. 



The Origin of New Species. — How, then, do new species 

 arise, supposing the world to have been, physically, much 

 as we now see it ; and what becomes of them after they 

 have arisen ? In the first place we must remember that 

 new species can only be formed when and where there is 

 room for them. If a continent is fully stocked with 

 animals, each species being so well adapted for its mode of 

 life that it can overcome all the dangers to which it is 

 exposed, and maintain on the average a tolerably uniform 

 population, then, so long as no change takes place, no new 

 species will arise. For every place or station is supposed 

 to be filled by creatures in all respects adapted to sur- 

 rounding conditions, able to defend themselves from all 

 enemies, and to obtain food notwithstanding the rivalry of 

 many competitors. But such a perfect balance of 

 organisms nowhere exists upon the earth, and probably 

 never has existed. The well-known fact that some species 

 are very common, while others are very rare, is an almost 

 certain proof that the one is better adapted to its position 

 than the other; and this belief is strengthened when we 

 find the individuals of one species ranging into different 



