iv MOISTURE AND DARKNESS OF COLOUR 97 



which produce this character and further examples of the 

 inheritance of acquired characters in general. 



"Leydig has pointed out that variation towards greater 

 darkness of colouring, the tendency to become black, is con- 

 nected with the action of moisture in the environment, having 

 observed this connection in the case of the dark variety of 

 Lacerta vivipara, of Amphibia, 1 and above all, of Arion 

 empiricorum. 2 He observed that, besides Arion empiricorum, 

 other molluscs, e.g. Helix arbustorum, Succinea Pfeifferi, 

 Helix circinata, become darker than usual in moist localities. 

 In Arion the colour even changes in this way according to 

 the dampness or dryness of the weather : ' In early spring, 

 when the ground and the air are still very moist, all the slugs, 

 in places where later on in the year only reddish-yellow 

 specimens are seen, have a dark brown colour.' For instance, 

 in the cool and rainy May of 1873 and in June with cold 

 weather and heavy rains the slugs in the saturated forest 

 of the Spitzberg (near Tubingen) were of a deep black. 

 Numerous other instances of the dark colour of Arion 

 empiricorum in moist places are given by Leydig. I can 

 confirm his statement that this slug under otherwise similar 

 conditions exhibits a dark colour in moist localities ; I found 

 that in the extremely damp summer of 1879 all specimens 

 were dark in places where at other times they were rather 

 light." 



"Some years ago, however, I pointed out a relation be- 

 tween elevation of habitat and darkness of colour in Arion 

 which is inconsistent with the supposition that moisture has 

 an absolute influence in producing this character." 3 I observed, 

 for instance, that Arion empiricorum on the heights of the 



1 Leydig, Die anuren Batrachier, Bonn, 1877. 



2 Leydig, Die Hautdecke und Schale der Gasteropoden, Arch.f. Naturgeschichte 

 von Troschel, 1876. 



3 Wurttembergische naturw. Jahreshefte, Vortrag gehalten im Verein fur 

 vaterl. Naturkunde zu Tubingen, 1878. 



H 



