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sibly it might be landed on one of the banks or arrested by some projecting object, so as to be able to crawl away in safety. Previously I had noticed that the level of the water had risen rapidly, and the scum, lying, no doubt, earlier in the evening stranded upon the mud at the water's edge, had evidently be enfloated off with its living burden by the "flood." Slugs of the same kind were numerous along the muddy margins for a considerable distance. The smaller ground-slugs, no doubt, like little shells of many kinds, often resort to the hollow "kexes" of umbellifers, which, as already mentioned, seem likely to serve as vehicles for transportal: the marsh-slug (L. lævis) has been noticed in the stems of these plants by Mr. B. Hudson,' and I have seen the field-slug (L. agrestis) in such stems lying upon the ground near a little water-course in Highgate Woods. Kinds which habitually burrow into the ground and spend a great part of their lives beneath the surface seem likely, of course, to be occasionally transported, at least in some regions, in the soil attached to the roots of floating trees, but they are sure to escape, in a great measure, the various accidental causes which above ground bring about comparatively frequent involuntary migrations; thus we find Dr. Simroth regarding the Testacella as having but little scope for dispersal owing to their subterranean habits, and as a consequence, he says, they have divided up into local forms.2
'Journ. of Conch.," v. (1886), p. 48.
2 H. Simroth, "Journ. of Conch.," vi. (1891), p. 423.
It cannot be supposed that slugs often cross the sea, and trans-oceanic migration for long distances, if ever occurring, is likely to be extremely rare. Dr. Scharff has stated, indeed, that the sea, " which is the principal means of communication for other animals, and plants, between mainland and island, forms an almost impassable barrier for slugs, sea water being deadly both to their eggs and themselves;" I do not think it necessarily safe, however, to assume, as that author does, that "if we find the slugs of mainland and island agreeing in anatomical characters, we may generally conclude that the island must have had a land connection with the mainland at some time or other," for this seems to imply that the creatures are incapable of crossing the ocean, and it is a fact, as I hear from Dr. Scharff, that their chances of surviving a sea passage are regarded by him as practically nil. But what do we actually know of the ways in which the various organisms are or have been distributed over the globe? Unthought-of means, even for the dispersal of slugs, may be in operation, or may have operated in former times. I have not ascertained whether any of the true oceanic islands, independently of man's dispersal, possessed slugs belonging to continental species or genera, but such I presume was the case; some kinds certainly seem to enjoy fairly wide ranges, but in many cases I suppose it would be difficult to ascertain to what extent man has affected their distribution. Few land
1 R. F. Scharff, "Sci. Trans. Royal Dublin Soc.,” (2), iv. (1891), p. 513.
molluscs, it is said, have attained so wide a range as our own little marsh-slug (Limax lævis), which, "under different names by various authors," has been recorded from Europe, North and South America, the West Indies, Madagascar, Australia, New Zealand, and many islands of the Pacific. Possibly or probably it has been carried to some of these regions by man, but it is interesting to find that in Australia, for instance, where it occurs "in very out-of-the-way places, and far away from the coastal cities," it is regarded as truly indigenous.2 Very probably, as it appears to me, these creatures are able in some way or other to journey over the sea-no doubt rarely-for short distances at least, and it seems possible also that accidents leading to transit over the ocean for very considerable distances may have occurred now and then in the course of the vast periods during which these naked genera have inhabited the earth, but we are unable, it is true, to refer to any actually observed occurrence that can reasonably be said to lend direct support to this view. The creatures' eggs, it will be remembered, do not differ essentially from the eggs of many snails. Those of certain kinds are deposited in the trunks of hollow trees; thus, for instance, Dr. Scharff mentions having found ova of the yellow-slug (Limax flavus) "in an old tree trunk, near Dublin," and those of the tree-slug (L. arborum), I
C. Hedley," Proc. Lin. Soc., N.S.W.," December, 1890, as quoted in the "Nautilus," v. (1891), 12.
2 C. T. Musson," Proc. Lin. Soc., N.S.W.," for 1890, (2), v. (1891), p. 885.
3 Sci. Trans. Royal Dublin Soc.," (2), iv. (1891), p. 522.
suppose, are usually placed either in the hollows or under the bark of trees. In such situations, protected, or partially protected, for a time from contact with sea water, ova might possibly be carried with floating trees over small arms of the sea, and perhaps even to islands, though rarely if ever to very remote ones. The animals themselves thus hiding might also be carried, I think, in a similar way. It is just possible, also, that eggs may be occasionally carried over the sea, at least for short distances, in the cavities of pumice, or in earth at the roots of trees. Large numbers, both adults and ova, must certainly be carried out to sea by large tropical rivers with floating rafts or islands, which have already been referred to, and if such objects have ever been stranded upon more or less distant shores so as to permit their inhabitants to be "poured out as from an ark," which was regarded as possible by Lyell,' any number of slugs may have been thus transported. Hurricanes and whirlwinds may have carried the creatures and their eggs (sometimes laid amongst dead leaves) over straits and small arms of the sea, and the just-hatched young are as likely as those of snails to crawl upon the feet of ground-roosting birds, and thus possibly they may be carried over sea, provided, of course, that they adhered firmly; this may be the case at a very early age, but later, I find, and more especially when young than adult, the creatures often let themselves drop purposely,
'See "Principles," ii. p. 366.
either with or without a slime-thread, from small objects upon which they may be placed, and young slugs, it should not be overlooked, soon perish from exposure, so that after a little time," even if they adhered, they would be sure to dry up and die. Dr. Scharff raises a further difficulty; supposing that a couple of young slugs, he writes, happened to stick to the feet of a bird and were carried a long distance without being shaken or blown off-which in itself he regards as unlikely enough-on being safely deposited on the ground they would “still have to keep together until mature, and not lose sight of one another in order to propagate their species." The difficulties constantly attending the establishment of new colonies as the result of all occasional means of dispersal, it must of course be admitted, are very great, and this fact, as already insisted, must never be lost sight of, but in connection with the present point it may be useful to remember that many kinds of birds migrate in immense numbers year after year along the same lines. To give an imaginary case: hundreds of migratory birds, let us suppose, roost together among tufts of herbage in a swamp, where slugs, hatching out from the egg, and abounding on all sides, adhere in numbers to their feet ; proceeding on migration, after a few hours, over an arm of the sea, they might deposit on the opposite shore a number of individuals, which, joining others similarly transported in former years, might become firmly established in the new home; or losing their way in a fog or otherwise, the birds might chance to alight altogether on some