THE ORIGIN OF SPECIES 65 



species is confined, and which is termed its area of distribution. 

 The members of different races will interbreed freely with one 

 another and produce fertile young ; nevertheless, in some cases 

 at any rate, the offspring tend to revert to the maternal and 

 paternal types, and in this way we are reminded of the rare cases 

 where two species will interbreed and produce fertile offspring. 

 Further, in most cases he reminds us, we are in great ignorance 

 as to whether two distinct types of animal or plant will or will 

 not unite with each other so as to produce fertile offspring 

 In few cases has the actual experiment been tried, and the 

 vast majority of distinctions between species which closely 

 resemble one another rest merely on the exhibition by all the 

 individuals of each species of some constant mark or character 

 which those forming the other species do not possess. So it 

 comes about that what one good authority regards as merely 

 distinct races another regards as distinct species. Now no one 

 has ever suggested that all the different races into which each 

 species is divided have existed since the beginning of life on 

 this globe; they are universally admitted to have originated 

 through changes produced in members of the mother species by 

 the different conditions to which they have been subjected in 

 different regions. The strong suspicion is therefore aroused 

 that distinctions between species themselves are of the same 

 nature as distinctions between races only carried to a further 

 extent, i.e. that races are in fact only species in the making. 

 This suspicion would rise to a certainty if we could show 

 that there is on the one hand some natural process going 

 on which tends to make the individuals of the same species 

 differ from one another, and if we could further show that 

 the property of sterility between two races when crossed could 

 gradually appear. These are the two propositions which Darwin 

 endeavoured to prove. He pointed out that no two members 

 of the same brood of chickens, no two puppies of the same 

 litter, were exactly alike, and that the differences between them 

 were often inheritable. He asserted that breeders of domestic 

 animals produced new " strains," i.e. new races of animals, 

 by selecting from the litters of their stock those individuals 

 which exhibited some particular trait which they wish to 

 perpetuate. When such individuals were mated together, they 

 often produced offspring in which the desired trait appeared 



