394 THE AMERICAN NATURALIST [Vol. LI 



pose a group of nothing but such hybrid individuals we 

 will get an enormous amount of variability in succeeding 

 generations, and when the group gradually becomes more 

 and more pure for an own genotype this may be a com- 

 pletely new one. A species may have been produced with 

 totally new characters, possibly intermediary between the 

 parent species in some of them. The chance that hybrids 

 of allogamous organisms, even if they are viable and 

 perfectly fertile, will inter se produce a new species is 

 exceedingly small in nature. It is much more probable 

 that the process of species formation after crossing is as 

 follows: 



There exists a species A, with a restricted potential 

 variability, a set of habits and mode of living all of its 

 own, adapted to a certain environment. As a general 

 rule, individuals of this species A mate exclusively with 

 members of their own species. Once in a while, small 

 groups may split themselves off from the multitude by 

 colonization, and each of these groups will have its own 

 potential variability, and each will gradually become pure 

 for its own genotype, and will be less variable than the 

 multitude. 



In the same country there exists a species B, with a 

 slightly different genot\T)e, a different potential variabil- 

 ity. Species B is somewhat differently built, somewhat 

 differently coated, compared with A, and therefore fits 

 into a somewhat different environment. As a rule, indi- 

 viduals of the two species do not come into touch. Let 

 us take as examples the grey-bellied Mus alexandrinum 

 which lives in bouses and on roofs in northern Africa, 

 and the white-bellied Mus tectorum, which lives in trees 

 in the same countries. The same holds true for the house- 

 rat and the field-rat in Java, likewise for the house-rat 

 and the tree-rat. 



Even if matings l)etween the two species furnish hybrids 

 which are completely fertile, even in localities where two 

 species overlap and are plentiful, the occasional hybrids 

 will be far in the minority compared to individuals pro- 



