SELECTION. 183 



are heavy. Individuals with a new genotype, which result from 

 a temporary heightening of the potential variability of the 

 species by a cross, have no chance to change the type of 

 the species, or to found a new species in the midst of the 

 old one. 



New, isolated colonies of plants or animals will simply be 

 colonies of members of existing species, unless the isolated 

 group should happen to have a markedly high potential varia- 

 bility. For the formation of a new species, no matter of how 

 ephemeral a standing, a combination of two circumstances is 

 necessary, colonization, or some other cause making for isola- 

 tion, and a typical variability of the isolated group. 



Cases in which we can notice this combination of two rela- 

 tively rare circumstances, have been observed not to infre- 

 quently. They can be said to be instances of the origin of spe- 

 cies. In most cases the causes for isolation are not of a perma- 

 nent nature, and there the newly formed species has only an 

 existence of very short duration. Two instances will illustrate 

 what I mean. 



Colonies of the Norway rat are being formed almost inevi- 

 tably where conditions of shelter and food-supply happen to be 

 favourable. These colonies consist almost always of normal, 

 typical Norway rats. But at least one instance has become 

 known of a colony of aberrant rats, black instead of grey, 

 which show the white markings on feet and belly of Mus 

 norvegicus much more plainly than the typical rats. This colony 

 was observed in Ireland in Co. Wexford by Rev. R. Keating. 

 It was a species of very temporary existence as a wild species, 

 but as a domestic one it is still being bred in cages by rat- 

 fanciers under the name of the "Irish" rat. 



Another similar case is the occurence of a species of very 

 marked black rats, Mus rattus, which were found to exist on 

 board a steamer going from Buenos Aires to Amsterdam. 

 It is probable that this species developed as such, on board 

 the steamer, possibly out of some hybrids, or from rats which 

 came on board at different ports. These rats were very small, 



