98 ACQUIRED CHARACTERS SEC. 



Kauhe Alb {e.g. above Urach), where there is little water, 

 was generally darker than at its base in the well -watered 

 valleys. Down the descent into the latter the animals 

 became lighter and lighter; and I found afterwards that 

 on all mountains on which I examined this species the 

 greater number of the specimens, or even all, were dark, 

 almost black, e.g. on the Schwarzwald, on the Harz, on the 

 Kigi, etc. 



Leydig states, on the contrary, that he found almost ex- 

 clusively Arion rufus on the dry heights of the Alb, 1 but my 

 observations are confirmed by another excellent observer of 

 molluscs Weinland, who found that Arion on the heights of 

 the Alb near his own home was usually dark, and never 

 reddish yellow as it so often is in the valley. " It might 

 be said," adds Weinland, " that darker pigment is always 

 produced on mountains as in Vipera prester, the black 

 mountain variety of Vipera berus, as in the black rattle- 

 snake of the "White Mountains in North America." " But," 

 he continues, " the law does not always hold : near the 

 Hohe Neuffen I found almost exclusively light -red speci- 

 mens." He attributes, therefore, the difference of colour to 

 adaptation, in which I am unable to agree with him. 2 The 

 explanation of the contradictions is probably that both 

 moisture and elevation produce darkness of colour both 

 causes may act together, and probably often do on mount- 

 ains, or great dryness may partially counteract the in- 

 fluence of elevation. More dark slugs ought to be found on 

 the heights in wet than in dry summers. 3 It must also be 

 pointed out that in many localities all colours occur side by 

 side, which can only be explained by the individuality, and 



1 Op. cit. separate copy, p. 60. 



2 Weinland, Zur Weichthiet fauna der SchwaUschen Alb, Jahreshefte des 

 Vereins fur vaterl. Naturkunde in Wilrttemberg, 1876. 



3 Only black specimens of Arion are said to occur at Hamburg. Perhaps the 

 moisture always produces this colour on the sea-coast. Compare the following. 



