CHAP. IV EVOLUTION THE KEY TO DISTRIBUTION 57 



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 will probably die out altogether, and thus leave 

 room for others to increase, or for new forms to occupy 

 their j)laces. 



But the change will most likely affect even flourishing 

 species in different ways, some beneficially, others inju- 

 riously. Or, again, it may affect a great many injuri- 

 ously, to such an extent as to require some change in their 

 structure or habits to enable them to get on as well as be- 

 fore. Now " variation " and the " struggle for exist- 

 ence " come into play. All the weaker and less perfectly 

 organised individuals die out, while those which vary 

 in such a way as to bring them into more harmony with 

 the new conditions constantly survive. If the change 

 of conditions has been considerable, then, after a few 

 centuries, or perhaps even a few generations, one or more 

 neio sjMcies will be almost sure to be formed. 



Variation in Animals. — To make this more intelligible 

 to those who have not considered the subject, and to 

 obviate the difficulty many feel about "favourable 

 variations occurring at the right time," it will be well to 

 discuss this matter a little more fally. Few persons 

 consider how largely and universally all animals are 

 varying. We know, however, that in every generation, if 

 we could examine all the individuals of any common 

 species, we should find considerable differences, not only 

 in size and colour, but in the form and proportions of all 

 the parts and organs of the body. In our domesticated 

 animals we know this to be the case, and it is by means of 

 the continual selection of such slight varieties to breed 

 from that all our extremely different domestic breeds have 

 been produced. Think of the difference in every limb, and 

 every bone and muscle, and probably in every part, 

 internal and external of the whole body, between a grey- 

 hound and a bull-dog ! Yet, if we had the complete series 

 of ancestors of these two breeds before us, we should prob- 



