SELECTION. 185 



multitude, will become pure for a genotype which will afford 

 the individuals having it a possibility to live in somewhat dif- 

 ferent conditions as compared with the related species. In 

 other words, it is possible for a small group of organisms with 

 a high potential variability, to come to fit into a different 

 ecological niche. This, evidently, is what we should term 

 natural selection. 



If a few seeds from hybrids between Lychnis diurna and L. 

 vespertina happen to reach a wet, shady, swampy, spot, it may 

 be thought possible, that one or two plants of this lot would 

 have a genotypic constitution which made life in these sur- 

 roundings possible. We know how greatly variable in every 

 point such progeny of species hybrids is. In this way under 

 the influence of natural selection, a new Lychnis species could 

 be thought to originate, for in those surroundings, the group 

 would be so effectively cut-off from inter-breeding with either 

 the multitude of Lychnis vespertina individuals, or with the 

 multitude of Lychnis diurna plants, that it would have a 

 chance to work out its own destiny as a distinct species. 

 This view of the action of natural selection at the time of 

 the origin of new species is wholly consistent with the facts 

 pointed out by Wagner and later again by Jordan, namely 

 that very closely related species are found, either separated 

 by geographic barriers, or in distinct "ecological niches", 

 and that only those groups of species coexist in one "ecologi- 

 cal niche," which do not inter-breed. 



To cite a few examples: The field-rat of Java has been 

 proved in our breeding-experiments to be at least geno- typically 

 different from the Sumatra field-rat. All over Java there is only 

 one field-rat, only one tree-rat, and only one big house-rat, 

 all closely related, and occasionally inter-breeding, but fitting 

 different ecological niches. 



The small house-rat and the big house-rat, Mus concolor and 

 griseiventer do occur together in the same houses, and they 

 live identically the same life. These rats, though obviously 

 closely related, remain separate species because of their differ- 



