94 THE VARIATION OF ANIMALS IN NATURE 



is confined to the Scilly Isles, where it is rare. It 

 is also known from Corsica and certain parts of 

 the Mediterranean. The type-form ranges all over 

 Europe. 

 9. The greenish female variety (var. valesina) of the Silverwashed 

 Fritillary, Argynnis paphia. — Goldschmidt (1922) has 

 shown that the variety is the expression of a single 

 dominant sex-limited gene. In England the variety 

 is confined to the New Forest, though the species 

 has a much wider range. The variety also occurs 

 sporadically on the Continent. 



10. The ' blue ' and ' white ' phases of the Arctic Fox [Elton, 

 I 93°> P- 8° and foil.). — These two forms often exist 

 together and interbreed with perfect fertility. The 

 proportions in which they occur are subject to much 

 local variation. In certain areas one or the other 

 form is found exclusively. 



11. Colour phases in birds. — Stresemann (1925) records in 

 birds a type of variation much like that seen in 

 the Arctic Fox. Thus the Indo-Australian Accipiter 

 novaehollandiae occurs in a white and a dark form. 

 In Tasmania, however, only the white form is found. 



B. Polymorphism. — This term has been applied, as we 

 have shown (p. 11), to variation in general and also in a 

 more restricted sense to the occurrence of strongly marked 

 phases within a species, whether they are geographically 

 distinct or occur in the same habitat. We propose to use the 

 term in the latter sense and to use ' Geographical Variation ' 

 for the occurrence of geographically isolated groups. 



1 . Colonial divergence in land snails. 



A great deal of intensive study has been devoted to the 

 statistical investigation of ' colonial ' divergence in land 

 mollusca. As the results are of considerable value we give 

 a more detailed analysis than usual and provide a summary 

 of the results. 



(a) Alkins (1928) studied two characters (altitude and 

 major diameter of shell) in Clausilia rugosa and 

 C. cravenensis in 19 loci distributed over an area of 

 8x4 miles. 



