88 THE VARIATION OF ANIMALS IN NATURE 



apparent. The same method may be applied to discontinuity 

 in a complex of characters, by means of a table showing the 

 extent to which they are correlated with one another. In 

 simple cases it may not even be essential to apply actual 

 statistical calculation. It is hardly necessary to point out 

 that discontinuity may be found between single characters 

 and between groups of characters and that, as Robson (1928, 

 p. 11) has shown, the attempt to formulate an exact standard 

 of specific distinctness based on the degree of discontinuity 

 in structural characters breaks down on account of the very 

 varying number of characters which may show discontinuous 

 differences. 



The question which really affects our present discussion 

 is the cause of this continuity and discontinuity of variation 

 and its usual mode of occurrence in nature. These two 

 subjects have been discussed by Bateson (191 3) and Robson 

 {I.e. p. 28 and foil.), and the following brief statement of their 

 views may be given with some expansion. 



Intermediate forms may be of two kinds — (i) ' mid-inter- 

 mediates,' which are a blend of the characters of two divergent 

 groups and represent a condition half-way between the two, 

 and (ii) various combinations of the characters of the two 

 groups. The former may be due to environmental causes or 

 to such genetic phenomena as imperfect dominance. The 

 latter are almost certainly due to genetic causes. Where a 

 genetic basis for intermediacy between species is involved, 

 it must arise from crossing or the intermediates may represent 

 the residuum of a stock from which distinct groups are being 

 evolved. It should be noted that between two species which 

 occupy the same area there may be intermediacy in one 

 region and none in another. This is noted for Cepea hortensis 

 and nemoralis by Coutagne (1895) and for Notonecta by Delcourt 

 (1909). It is even seen in such a restricted area as a single 

 lake, as has been recorded in the pond snail Vivipara of Lake 

 Garda by Franz (1928). The extent to which intermediacy 

 in nature is brought about by crossing is very uncertain. That 

 a great deal has the appearance of being due to this cause is 

 undoubted, and many systematists (e.g. Pictet, 1926, p. 399 ; 

 Ruxton and Schwarz, 1929, p. 571 ; Lowe, 1929, p. 29) are 

 of the opinion that particular intermediate populations are 

 produced by this cause. Crampton (1932, p. 160 and passim) 



