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of snail or slug is a necessary factor in the causation of rot. Snails and 

 slugs are but scantily represeVited, if present at all, on mountain tops ; 

 there are no slugs in Forbes' and Tschudi's lists of high Alpine ranges; 

 and these districts, like salt marshes, owe their character for considered 

 »soundness«, as the phrase is as regards the most destructive of sheep 

 diseases, to this abscence or paucity of, at least, certain mollusca, 



I was first put in this pursuit upon the slimy trail of the slugs and 

 snails specified by various well-known facts which it is here unneces- 

 sary to specify. The number of the mollusca which it is necessary to 

 trouble about appears to me to be very distinctly and very conveniently 

 limited by the fact pointed out by the late Willemoes-Suhm, as to the 

 Faroe Islands. The Faroe Islands are afflicted by the rot, but they have 

 only eight snails and slugs, all told, out of which to choose the guilty 

 party or parties. These are, as enumerated in Siebold and Kölliker's 

 Zeitschrift for 1873 — vol. XXIII, p. 339 — Limax agrestis, Limax 

 margmatus , Vitrina pellucida, Hyalina alliaria^ Limnaeus per eg er, 

 Limnaeus truncatulus, Arion ater, Arioti cinctus. That the fluke of the 

 Faroe Islands sheep spends a considerable part of its life as a parasite 

 in one or more of these mollusca admits of about as much doubt as the 

 statement that the »giddy« disease, the »sturdy« as the Lake District 

 shepherds pronounce the French word , étourdi', of the sheep is simi- 

 larly dependent upon a to-and-fro shuttlecock alternation of one animal 

 between two others. I do not say that it is necessarily in one, and one 

 only, of the specified eight mollusca. Undoubtedly the fluke, like other 

 parasites, may, in its sporting tour, infest many hosts. As a matter of 

 fact, this particular fluke [Fasciola hepatica) in its adult stage infests 

 some dozen mammals besides the sheep and ourselves. But, as a matter 

 of speculation, I incline to think that the fluke in its younger days is, 

 like some other animals, a little, or indeed a good deal, more particular 

 as to where it lodges than it is in later life. As a matter of practice, 

 at any rate, there is no need to tell farmers to be on their guard against 

 snails which do not infest their pastures, and of the eight just specified 

 they need, usually in England, only look to the black slug and the gray 

 slug. Willemoes-Suhm suspected the gray slug. I suspect the black slug, 

 partly on account of its very wide distribution in space partly from the 

 facts furnished to me by Mr. D. Gresswell, of Louth, to the effect that 

 sheep which have been feeding on turnips harbouring »black jacks« will 

 die of rot even when shifted on to salt marshes. Any inhabitant of the 

 Shetland Islands who will inform us that the sheep-rot does infest his 

 country, or any inhabitant of Siberia who will inform us that it does 

 not infest his, will settle the question, in a preliminary way at least, in 

 favour of the black slug. For »this very common, beautiful, and ex- 



