A very singular case of the reverse process, _i.e._ the production of darkened forms of shell through cold, has been noticed by Fischer as characteristic of the marine shells of the west coast of South America.[186] This melanism is especially noticeable in _Trochus_, _Turbo_, _Chiton_, _Mitra_, and _Pleurotoma_, and is attested by the specific names, not merely expressive of actual blackness (_e.g._ _nigerrimus_, _ater_, _atramentarius_, _maurus_), but also of a generally lugubrious tone (_e.g._ _moestus_, _funebralis_, _tristis_, _lugubris_, _luctuosus_). It is highly probable that this concurrence of specific melanism (which stands quite alone in the world) is 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 _Arion 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 Pollonera, the length of the keel in _Limax arborum_ 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, _Arion ater_, is a northern type, which 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.”[187] According to Scharff, however[188] (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.