OF SPECIES 39 



of life, they become subject to influences which little by little alter the 

 consistency and proportions of their parts, their shape, properties and 

 even their organisation ; so that in course of time everything in them 

 shares in these mutations. 



In the same climate, very different habitats and conditions at first 

 merely cause variations in the individuals exposed to them ; but 

 in course of time the continued change of habitat in the individuals 

 of which I speak, living and reproducing in these new conditions, 

 induces alterations in them which become more or less essential to 

 their being ; thus, after a long succession of generations these in- 

 dividuals, originally belonging to one species, become at length 

 transformed into a new species distinct from the first. 



Suppose, for example, that the seeds of a grass or any other plant 

 that grows normally in a damp meadow, are somehow conveyed 

 first to the slope of a neighbouring hill where the ground although 

 higher is still rich enough to allow the plant to maintain its existence. 

 Suppose that then, after living there and reproducing itself many times, 

 it reaches little by little the dry and almost barren ground of a mountain 

 side. If the plant succeeds in living there and perpetuating itself 

 for a number of generations, it will have become so altered that botanists 

 who come across it will erect it into a separate species. 



The same thing happens in the case of animals that are forced 

 by circumstances to change their climate, habits, and manner of 

 life : but in their case more time is required to work any noticeable 

 change than in the case of plants. 



The idea of bringing together under the name of species a collection 

 of like individuals, which perpetuate themselves unchanged by re- 

 production and are as old as nature, involved the assumption that the 

 individuals of one species could not unite in reproductive acts with 

 individuals of a different species. 



Unfortunately, observation has proved and continues every day to 

 prove that this assumption is unwarranted ; for the hybrids so common 

 among plants, and the copulations so often noticed between animals 

 of very different species, disclose the fact that the boundaries between 

 these alleged constant species are not so impassable as had been 

 imagined. 



It is true that often nothing results from these strange copulations, 

 especially when the animals are very disparate ; and when any- 

 thing does happen the resulting individuals are usually infertile ; 

 but we also know that when there is less disparity these defects do not 

 occur. Now this cause is by itself sufficient gradually to create 

 varieties, which then become races, and in course of time constitute 

 what we call species. 



