CHAP. IV.] EVOLUTION THE KEY TO DISTRIBUTION. 



55 



exactly the same general phenomeDa 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 compre- 

 hensive 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 twenty years ago, 

 and have now come to be adopted by naturalists as established 

 propositions in the theory of evolution. 



The Origin of Neio 8pecies. — 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 perfectly adapted to all surrounding 

 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 climates, subsisting on different food, and competing 

 with different sets of animals, while the individuals of another 

 species will be limited to a small area beyond which they 

 seem unable to extend. When a change occurs, either of 

 climate or geography, some of the small and ill-adapted species 



