chap, iv.] EVOLUTION THE KEY TO DISTRIBUTION. 
55 
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 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 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 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 
