THE DISTRIBUTION OF VARIANTS IN NATURE 93 



3. ' Xanthochroism ' in fishes. — The black and brown pig- 

 ment is lost more or less entirely in certain groups 

 and the golden and yellow is left. The Goldfish is, 

 of course, a cultivated variant of this type. This 

 condition is found in nature in the Trout and Eel 

 (Norman, I.e. p. 227). 



4. Variation in sculpture in the water beetles, Dytiscidae 

 {Kolbe, ig2o). — In many species two forms of the 

 female occur, a smooth form and a sculptured form. 

 The latter may have deep striae or merely denser 

 microscopic sculpture, according to the genus and 

 species. In most cases the proportion of the types 

 varies locally and one or other form may be found 

 almost exclusively in certain parts of the range. 



5. Colour and pattern forms of land snails. — Many species 

 of Helicidae are extremely variable in colour and 

 pattern. It seems at present that the variation is 

 subject to some measure of local statistical divergence ; 

 but certain pattern combinations are rare and occur 

 in single individuals in most local assemblages. 



6. Sinistral varieties of normally dextral snails {Crampton, 

 igi6, ig2j and 1332). — These are somewhat similarly 

 distributed, but are normally rare, while areas of 

 high frequency are very localised. 



7. Colour variation in the wasp, Synagris cornuta L. 

 (Beguaert, igig, p. 204). — The species is practically 

 confined to Engler's Western Forest Province of 

 Africa. There are eight distinct colour forms. 

 Many of these occur together in any one district and 

 several of them have been found in a single nest. 

 Many intergrades occur. The ground colour is black 

 with black wings, and variation consists in the pre- 

 sence or absence of varying amounts of orange on the 

 thorax and base of the abdomen. These occur in 

 all combinations. 



(b) There is a typical form and a variety or varieties are 

 localised in definite parts of the range, where they 

 occur with the typical form. 



8. The black variety (var. nigra) of the Rose Chafer, Cetonia 

 aurata {Blair, igy, p. 121 2). — In Great Britain this 



