56 ANIMAL COLOEATION. 



All these facts are clearly in harmony, and indicate a direct 

 variation of colour in correspondence with changes in tem- 

 perature, which would be very difficult to be proved useful to 

 the insects, and therefore perpetuated by natural selection. 



Professor Leydig (quoted by Eimer*) gives an interesting- 

 account of certain colour changes which are apparently pro- 

 duced by greater abundance of moisture in the atmosphere. 

 " The influence of light and warmth," he says, " is very 

 distinctly evident in the colouring of Helix nemo rails in this 

 region. The fine citron vellow exhibited by the shell of this 



O f */ 



snail at Mainz, and in the sunny vineyards of the Main valley, 



is wanting on the banks of the Lower Rhine On the 



other hand, it is interesting to notice how in the neighbourhood 

 of Bonn and lower down the Rhine the red of this snail deepens 

 into chocolate brown, and thus forms the beautiful variety 

 above mentioned, which must attract the attention of every 

 collector. It is, perhaps, too much to say that the moisture of 

 the plain of the Lower Rhine determines this change of colour, 

 but we must keep in view the possibility that the moisture 

 coming up the valley from the sea, which certainly here at 

 Bonn has a distinct effect on the vegetation, has something to 

 do with the matter." The colours of snail-shells, so varied as 

 they are and frequently so brilliant, can hardly have any other 

 signification, if we seek to attribute them to natural selection, 

 than " warning colours " or sexual. With regard to the latter 

 possibility sexual selection in hermaphrodite animals is a little 

 difficult to believe in ; arid Eimer has stated that a number 

 of snails in his garden, which he kept under observation for 

 some years, did not appear to exercise any choice whatever in 

 pairing. So far as concerns warning colours, the above instance 

 would have to be explained by the greater abundance of 



* " Organic Evolution," p. 137. 



