THE DISTRIBUTION OF VARIANTS IN NATURE 89 



evidently holds that there is good evidence that much of the 

 interracial intermediacy in Par tula is due to crossing. In a 

 later chapter we give an account of the factors that make for 

 isolation between species in nature, and it will be seen that 

 they are many and varied. Though it may amount to a 

 truism, we must content ourselves with the conclusion that, 

 wherever opportunities for crossing are available, a good part 

 of the intermediacy (notably in respect of recombinations of 

 characters) found in nature, is due to this cause. 



Although all degrees of intermediacy are found in nature 

 there are certain broad lines which can be recognised in their 

 mode of occurrence. 



Observations in nature suggest that there are three main 

 tendencies recognisable at the meeting-point of allied species 

 or races occupying distinct areas. 



(1) The groups occupy distinct areas with few or no 

 intermediates — Lepidoptera (Clark, 1932, p. 8), Peromyscus 

 maniculatus and P. blandus (Dice, 1931), Eumenes maxillosus 

 typicus and tropicalis and fenestralis (Bequaert, 1919). This 

 may occur either with topographical discontinuity (Thomas 

 and Wroughton, 191 6 (squirrels) ) or without (Dice, I.e.). 



(2) There is a narrow area between the two groups occupied 

 by an intermediate type — Tisiphone species (Waterhouse, 1922), 

 Passerella iliaca (Swarth, 1920), Peromyscus albifrons and P. polio- 

 notus (Sumner, 1929). It is interesting to note that the area 

 of intergradation is very narrow in the last-mentioned case, 

 although the species in question are known experimentally 

 to be quite fertile inter se (Sumner, I.e. p. 114). In the case 

 of Passerella the subspecies mentioned may be broken up into 

 separate populations {i.e. there may be no continuity of popu- 

 lation). 



(3) A number of subspecies may occur over a larger 

 or smaller area with complete intergradation between the 

 various groups — Troglodytes musculus (Chapman and Griscom, 

 1924), Heodes phlaeas (Ford, 1924). It is of importance to 

 note that these tendencies may be observed in one and the 

 same group. Thus Clark (1932, p. 8) states that ' while 

 some species pass by a series of minute intergradations from 

 one geographical form to another, others do not, the N. and 

 S. form occurring together with one or perhaps two well- 

 marked intergrading types.' So, too, one may note the sharp 



