THE YOUNG NATURALIST. 



189 



cidae or the Slug, not for the reason that they are the easiest of any to work 

 at, but because they provide many examples of what a type is, and what a 

 variation from a type means. Perhaps, with the reserve of the Leech-group, 

 the snails and slugs are subject to more varietal marking than any other sub- 

 division of the zoological kingdom. What the real cause of this variation 

 may be, remains as yet veiled to us, and there can be but little doubt but that 

 we shall have to wait some time before we surely, and once for all, get at the 

 bottom of things. Perhaps external conditions, such as the interference of 

 the economy of the animal by wet, damp, or prevalent sunshine, may be a 

 factor in the production of the change ; then the difference of the food-plant 

 in one region to that in another ; and lastly, and in my mind the greatest 

 factor yet known, is cross-breeding — just as in the plant-kingdom we get 

 many instances what a great effect is worked by this cause in the making of 

 the beautiful flowers around us. That this last is no idle speculation, 1 have 

 proved over and over again by rearing slugs, and producing varieties by inter- 

 crossing, though I should be going out of my way indeed, to dogmatically 

 assert that this is the sole cause, and that we have to look for none other. I 

 simply assert that in my own thinking, and from my own observations it is 

 the most active cause so far as our knowledge extends at present. From 

 some force or other then slugs are varying in their markings at every genera- 

 tion that is born. We never see two generations from one parent alike. If 

 we had chronicled all the varieties on this earth to-day, that would be no 

 reason for us to state that we had done our work. One year, or two years 

 hence, some fresh arrivals in the field would occur, and additions would be 

 made to our already large list. So much is this so that we may almost 

 safely aver that every slug we find is a variety, and that we have no such dis- 

 tinctive characters as in other groups, to tell us what is this species or what 

 is that species. The real type of a species from which all the varieties have 

 originated we cannot tell, but because we must have types for description's 

 sake, we have picked out the one slug of a species whose characters seem to 

 be more fixed than any one other, and given it the name of type. Then if a 

 slug be found differing in colouration or what not, from the description of a 

 type, but in the main possessing all the specific distinctions of that type, then 

 it is a variety of that type. I hope this is plain, for the meanings of the two 

 terms " type " and " variety " must be well understood by every slug col- 

 lector, and by every one who would understand the current descriptions 

 of slugs. 



There are four genera of our slugs, Arion, Geomalcus, Limax, and Testa- 

 cella. The second of these, in this paper we may pass, over for it is exceed- 

 ingly rare, and has only been found in one or two places in Ireland. The 



