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THE AMERICAN NATURALIST [Vol. LI 



In one territory, two species can coexist only if for 

 some reason matings between individuals of different 

 species are impossible or at least less common than 

 matings between members of the same species, or when 

 the hybrids are sterile. 



lN^o matter where we find rats of the Rattus group there 

 are never more than one kind of tree-rat of this group, one 

 house-rat, and one field-rat simultaneously present in one 

 locality, the tree-rat living and foraging in trees, and 

 being exceptionally aggressive, the house-rat living in 

 houses, fearing water, and not afraid of man, the field- 

 rat living even far from cover, scarcely able to climb and 

 too timid to enter inhabited houses. In some regions 

 miniature rats, belonging to the same group, but too small 

 to mate with the bigger species as a rule, inhabit houses 

 and fields, it being very probable that these belong to two 

 species, a small field-rat and a small house-rat. 



Such a set of three rat species, a tree-rat, a house-rat 

 and a field-rat, we not only find in Java, but also on Su- 

 matra, the Malay peninsula, BritishTlndia and Egypt. 



It is our experience that rats of the Eattus group cross 

 with the utmost facility, and produce fertile hybrids. We 

 have come to the conclusion that the majority of house 

 rats remain pure for their own characteristics, even for 

 those which have no value whatever for the adaptation 

 of the species to its surroundings, not because no hybrids 

 are produced with tree-rats or with field-rats, or not be- 

 cause such hybrids when produced are sterile, but for the 

 simple reason that such hybrids are so far in the minority 

 that they disappear into the multitude of house-rats, and 

 that the heightening of the potential variability of the 

 house-rat multitade by such occasional crosses is only 

 local and very transitory. The same is true for the field- 

 rat multitude and the tree-rat population. Crossing pro- 

 duces for each of the three species a heightening of the 

 potential variability, and therefore it is possible tliat 

 more or less temporarily, there come into being small col- 

 onies of somewhat aberrant house-rats or field-rats, in 

 isolated places. 



