VARIATION AND LOCALITY 135 



The genetic relations of local forms to each other cannot in 

 the absence of actual breeding experiments be often ascertained. 

 Standfuss formerly enunciated as a general principle that when 

 two forms co-exist in the same locality and are able to interbreed, 

 they do not produce intermediates; but that when the forms are 

 geographically separated as local races, crosses between them 

 result in a series of intermediates. 20 In this aphorism there is a 

 good deal of truth, but if in the light of Mendelian principles we 

 examine the two statements we see now that the first is in reality 

 only another way of saying that the distinctness of an aberrational 

 form co-existing with another is due to segregation, accompanied 

 by some degree of dominance of one type. Whether, however, 

 one geographically isolated race will give intermediates when 

 bred with another must depend entirely on the genetic physiology 

 of the special case, and no general rule can be laid down. It 

 may well be that, inasmuch as the distinctness of the variety is 

 maintained by isolation, the difference in factorial composition 

 between it and the representative form in another area is neither 

 simple nor sharp; but when two varieties co-exist, though inter- 

 breeding, it is now clear that their differences must depend on 

 the segregation of simple factors. Plainly such aberrations may 

 in one place co-exist with another type, and elsewhere be sep- 

 arated from it as local races. 



Excellent illustrations of these two stages in evolution are 

 provided by the melanic varieties of British Lepidoptera. The 

 fact that black or blackish varieties of many species especially 

 of Geometridae have come into existence in recent years is well 

 known to British collectors, and it is not in dispute that they 

 have in several instances replaced the older type more or less 

 completely in certain districts. In the year 1900 the Evolution 

 Committee of the Royal Society instituted a collective inquiry 

 as to the contemporary distribution of these dark varieties. As 

 the change had happened within living memory and had greatly 

 progressed in recent years it was hoped that a record of the 

 existing distribution would serve as a point of departure for 

 future comparison. The records thus obtained were tabulated 



20 Standfuss, Handbuch d. palaarkt Gross-schmet, 1896, p. 321. 



