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human exhalations, too small in quantity to be discovered, 

 yet sufficiently powerful to produce most noxious effects 

 upon the constitution. Is it possible that some analogous 

 phenomenon is the cause of melanism, that some gas is 

 present in minute quantities in moist atmospheres, perhaps 

 produced by chemical combination from other and harmless 

 gases, and this is powerful enough to darken the pigment 

 of animals and produce melanism ? or should we attribute 

 it to the direct effects of moisture alone, or to some other 

 cause ? 



Examples under the head of melanism are quite familiar to 

 you, and I will not enumerate them, but mention only one 

 case that struck me a good deal when I first read of it. It is 

 the Arvicola ainpJiibia, var. ater MacGillivray, or the Black 

 Water Vole. This variety differs only from the type in being 

 black instead of brown, in fact, a melanic form. It occurs 

 commonly in some of the northern counties of Scotland, 

 where the brown typical form is rare ; while in the south of 

 Scotland and some parts of England it is found rarely, the 

 brown being the prevalent form in those districts. Surely 

 this variety, considering its geographical distribution, must 

 be classed with Scotch melanism generally, as due, probably 

 at any rate, to the same cause ; yet considering the habits 

 and nature of the animal, it is not very easy to imagine how 

 the same cause could have so influenced water-vole, slugs 

 and Lepidoptera. 



I have said so much about colour varieties that there is 

 hardly time to speak of varieties of form, size, and texture 

 and I will therefore be as brief as possible. Of form-varieties 

 one of the most curious is that of reversion of the' normal 

 position of the parts in species in which the sides are not 

 alike ; for instance, some men have their hearts on the right 

 instead of the left side, with the other organs correspondingly 

 reversed. Reversed flat fish are sometimes taken ; the pecu- 

 liarity has occurred for instance, in Solea znilgaris, Rhombus 

 IcEvis, R. maxinius, and Pleiironectes flcsiis. Shells of Gastero- 

 pods, in which the spire is dextral, will have sinistral aber- 

 rations ; and others, in which the spire is normally sinistral. 



