No. 607] RATS AXD EVOLUriOX 399 



standard. In silver Wyandottes, the standard calls for 

 white feathers, which are bordered by a line of black. 

 Now the males are very much lighter than the females, 

 so that in a pure species, in which the males are correetly 

 marked, the hens are too dark and in a strain in which the 

 hens are good, the males are too light. The only way out 

 of the difficulty has been the establishment of two dif- 

 ferent species, one which produces correct males and the 

 other which produces exhibition hens. This splitting up 

 of a species into two is very common in chickens. Such 

 pairs of two species are kept as far apart by careful 

 breeders as Wyandottes and Leghorns. 



In the natural state, two species, even when hybrids 

 between them are perfectly fertile, and when the indi- 

 viduals exhibit no preference for mating with their kind, 

 may keep apart, if only each group is specially adapted 

 to an own environment, so that the bulk of the animals of 

 each species has no chance of mating with an>i:hing but 

 their kind. 



Even if there is no adaptation to an environment to 

 keep the multitude of the individuals of a species in their 

 place, two species may keep apart when only the animal's 

 habits keep them from wandering very far. In such a 

 case the borderline, where outposts of both species mix, 

 will present a highly variable population of hybrids of 

 all grades, the variation becoming less and less the more 

 we look for the animals in the exclusive territory of each 

 species. A case in point is, we think, the case of the two 

 woodpeckers cited by William Bateson.'^ When the spe- 

 cies differ in only one salient characteristic, the difference 

 between them being in the main due to the presence or 

 absence of only one gene, intermediates must be absent. 

 In such a ease the two species may be present in more or 

 less extensive patches, separated from each other by 

 narrow strl])s of territory having a mixed population. 

 Such s(M"iii^ to })(' tile f'a>p of the black and the hooded 

 ci-ows ill l-:uit)|K\ HtM-." adaptation plays no role ap- 

 parently. 



