EVOLUTION IN NATURE AND UNDER DOMESTICATION. 247 



matings as compared with inter-specific ones. The two species 

 may differ in size, or they may react in a different way to the 

 seasons so that their mating periods do not coincide. 



In attempts to hybridize species, a difference becomes appar- 

 ent between the various sets of species found in nature. 



When two species coexist in one environment, and when 

 they are found to live the same life, and to occur in the same 

 spots, hybridization will for some reason be practically impos- 

 sible. It is apparent that we may look for sterile hybrids in 

 such cases, though we have not met which such an instance. 

 Very often in the case of animals, no matings occur. The case 

 of the two house-rats of Java and the Malay peninsula, Mus 

 griseiventer and concolor is a typical instance. 



Where local species of closely contingent regions are cros- 

 sed, they will mate as readily as members of the same group. 

 And the difference between them will be found to be compara- 

 tively slight, comprising a few genes. The cross-breeding exper- 

 iments of Sumner with Peromyscus have shown that the 

 genotypic difference between local forms is slight. 



Lang's experiments with snails, Helix, prove the same 

 thing. 



When two closely related species which inhabit the same 

 territory, but living a different life are tried, it will sometimes 

 be found to be almost impossible to cross them. Such a case 

 is that of the Javanese field-rat arid house-rat. In other cases, 

 such as that of the house-rat and tree-rat, hybrids are easily 

 produced from caged animals, which shows that the reason 

 for the specific distinctness must here be ascribed to the differ- 

 ent mode of life. In the case of the house-rat and the tree-rat, 

 the result of the cross shows that the genotype of the two spe- 

 cies is more different than between contingent local species. 

 In the second generation unexpected new characters are seen 

 in several animals, just as in the case of a cross between two 

 rather widely different species of plants. 



This difference in genotype between closely related species 

 occurring in different ecological niches in the same environ- 



