SECT, ix.] INTRODUCTION. 49 



may be so striking that each individual can at once be recognized 

 as belonging to a given locality, or they may be trifling, and 

 appreciable only when a large number of individuals are gathered. 

 But apart from these differences of form and texture there are a 

 great number of colour-varieties of which the following are the 

 three chief whole-coloured forms, viz. white, dark purple-brown, 

 and yellow. In addition to these there are banded forms, and the 

 bands may be coloured with any two of the three colours men- 

 tioned above. Among the banded forms there are two distinct 

 sorts of banding, in the one there are very many fine bands and in 

 the other there are a few broad bands. In most localities these 

 colour- varieties may all be found ; though in some places, especi- 

 ally where the water is foul, as at Plymouth, the shells are greatly 

 corroded and the colours, if originally present, are obscured. 

 Speaking however of localities in which colour- varieties are to be 

 seen at all, several may generally be found together. If any one 

 will take the trouble to gather a few hundreds of these shells and 

 will set himself to sort them into groups according to their 

 colours, he will find that the majority fall naturally into groups of 

 this kind ; and that those which cannot be at once assigned to 

 groups but fall intermediately between the groups are com- 

 paratively few. I have seen this at many places on the English 

 coast; in Yorkshire, Norfolk, Suffolk, Kent, Sussex, Dorsetshire, 

 Devonshire, Cornwall, &c. In several localities I have found 

 pairs belonging to different colour-varieties breeding together, 

 and there is therefore no reasonable doubt that these colour- 

 variations do not freely blend, but are discontinuous. 



The statements here made with regard to P. lapillus hold in 

 almost the same way for Littorina rudis, but in this case the 

 number of colour-types is larger. In L. rudis I have occasionally 

 seen specimens of which the upper part belonged to one colour- 

 type, and the lower to another, the transition occurring sharply at 

 one of the varices. In these cases the shell appears to have been 

 injured and is possibly renewed. 



One of the commonest British Lady-birds (Goccinella decern- 

 punctata) is an extremely variable form. A great number of its 

 varieties may be found together, ranging from forms with small 

 black spots on a red field to forms in which the field is black with 

 a few red spots. But in spite of the great diversity there are 

 certain types which are again and again approached, while the 

 intermediates are comparatively scarce. 



The following case, well known to entomologists, may be mentioned here. The 

 Painted Lady (Pyramels cardui) is found in the typical form over the entire extent 

 of every continent, with the exception of the Arctic regions and possibly S. America. 

 A special form of it (var. kershawi) is found in Australia and New Zealand, but the 

 other large islands south of Asia possess the normal type. The latter is also found 

 in the Azores, Canaries, Madeira and St Helena. This butterfly has been taken on 

 the snow-level in the Alps ; and in N. America, though it may be regarded as one 

 of the commonest butterflies in the elevated central district, it is most abundant at 

 a level of 7000 8000 feet. It has been taken on Arapahoe Peak, between 11,000 



B. 4 



