S6 VARIATION IN SLUGS chap. 



due to the cold polar current which impinges on the Chilian 

 coasts, for the same genera occur on the opposite shores of the 

 continent without exhibiting any trace whatever of this mournful 

 characteristic. 



It is a well-known fact, attested by many observers, that our 

 common Limax agrestis as well as the young of Avion ater 

 become decidedly darker in summer than in winter. If these 

 slugs were accustomed to disport themselves in the sun, it might 

 have been suggested that this increased darkness of colour 

 tended to absorb more of the heat rays. But since this is not 

 the case, the result is probably due to some unexplained effect of 

 higher temperature. According to Lessona and PoUonera, the 

 length of the keel in Limax arhorum varies greatly in different 

 parts of Italy, being shorter in specimens from low ground, but 

 much longer in those inhabiting more elevated regions. The 

 longer the keel, the more obscure the colouring becomes, so that 

 in the Upper Alps of Piedmont individuals are practically black. 

 Roebuck has observed that Scottish specimens of this same 

 slug are much darker and less translucent than English forms. 

 According to Simroth, our common black slug. Avion atev^ is a 

 northern type, w^hich in more southern latitudes assumes the 

 form known as A. rufus. Similarly Limax maximus *'in its 

 northern form cinereo-niger is almost wholly black, but in the 

 more genial climate of Italy develops a series of brilliantly 

 coloured and strikingly marked variations which have received 

 numerous distinctive names from Italian limacologists." ^ Ac- 

 cording to Scharff, however ^ (who regards the colours of slugs 

 as in the main protective), these dark forms are by no means 

 exclusively northern, being found equally on the parched plains 

 of Spain and Portugal, and in the bleak climate of Norway. The 

 same authority observes that similar forms occur both in the dry 

 regions of E. Germany, and in the very humid district of western 

 Ireland. 



It appears unquestionable that marine genera from high 

 northern latitudes are provided with shells of uniform colour, or 

 whitish with a pale brown epidermis ; spots, bands, or stripes 

 seldom occur. The arctic forms of Buccinum, Trophon^ Chryso- 

 domus^ Mavgavita, Crenella, Leda^ Yoldia^ Astarte illustrate this 

 fact. In the more temperate seas of Europe, colours tend on the 



1 J. W. Taylor ut siq). p. 300. 2 ^ci. Trans. M. Duhl. Soc. (2) iv. p. 555. 



