lone islet, at no great distance, all the moist surfaces of which, previously uninhabited by slugs perhaps, might possibly be found, after a few centuries, as some traveller might record, swarming with a kind common to the mainland. We are bound to conclude, however, I think, that in all probability dispersal over sea, with subsequent colonization, even to short distances, is very rare, and such dispersal over wide stretches, though perhaps not impossible, can hardly be supposed to have happened, by any known means, more than once or twice in the course of vast ages. One other point deserves attention. Slugs are known to be largely eaten by many birds; we find, for instance, that young chickens and ducks are sometimes employed with good results for the purpose of keeping the creatures within bounds in kitchen gardens; when the former are allowed the free range of a garden they may be seen, it is said, at daybreak, regularly searching rows of cabbages and broccoli, picking up slugs, etc.; plovers and gulls also, thus employed, are stated to be very useful, and a tame petrel is said to have proved valuable for the same purpose. If the creatures can survive in the crop for a few hours, it would certainly seem that they may be transported, as already suggested for snails, for short distances, and if they can survive say for twelve or eighteen hours, they may be carried over sea, in exceptional circumstances, according to an

1

"Garden," x. (1876), p. 435; xiii. (1878), pp. 275, 304, 350; xvi. (1879), p. 391 ; "Gardeners' Chronicle," 1878, p. 664; "Science Gossip," 1868, p. 46; etc.

estimate of Mr. Darwin's, even for a distance of 500 miles, and remaining alive in the crops of dead birds, they might be drifted on the surface of the ocean possibly to a still greater distance. The late Mr. J. W. G. Spicer, of Spye Park, Wilts, once recorded the shooting of pheasants in the Holt Forest with their crops completely full of a small white slug, but he gave no indication as to whether any were alive,' and perhaps it is hardly likely that these shell-less creatures can retain life for any considerable time in the warm crop of a bird, and it is to be remarked, also, that they are certainly more likely than snails to be killed or severely injured when picked up and swallowed.

1 J. W. G. Spicer, "Zoologist," (3), v. (1881), p. 383.

N

CHAPTER VIII.

DISPERSAL OF FRESH-WATER AND LAND MOLLUSCA BY MAN.

IN comparatively recent times many molluscs have been widely scattered over the globe by man. Indeed it may be safely said that both snails and slugs, during the last three centuries at least, have been more dispersed by human agency than by all other causes together. European species, for instance, thus disseminated, as everyone knows, have become thoroughly naturalized in many parts of the world, even in the most remote colonies. Many facts illustrating the extent to which certain faunas have been alloyed by man might be given. All the species of Limax described as native to Australasia are referable, according to Mr. Hedley, either to L. maximus, flavus, gagates, agrestis, or lævis, all, except the last, believed to have been introduced by man from Europe. The Madeiran Islands, to give another instance, discovered

1 See Woodward's "Manual," ed. 4, rep. 1890, p. 289.

2 C. Hedley, "Ann. and Mag. Nat. Hist.," (6), ix. (1892), p. 170; C. T. Musson," Proc. Lin. Soc., N.S.W." for 1890 (2), v. (1891), p. 885.

in 1418, sea-girt, and less than 400 square miles in area, possess, according to the Rev. Dr. Watson, no less than thirteen kinds of molluscs (six slugs and seven land and fresh-water shells), believed to have been introduced by man, and now well-naturalized, namely:Arion ater, Limax gagates, L. maximus, L. flavus, L. agrestis, Testacella maugei, Hyalinia cellaria, Helix pulchella, Bulimus decollatus, B. ventricosus, Cochlicopa lubrica, Limnæa acuta, and " Ancylus striatus Q. and G."; and there are also six other similarly introduced kinds-Testacella haliotidea, Helix aspersa, H. rotundata, Planorbis glaber, Physa acuta, and Hydrobia similis— much more recently imported, and hardly to be regarded as truly naturalized.' Snails thus introduced, as it is interesting to note, often increase with surprising rapidity; it is notoriously dangerous, indeed, to turn out any creature in a new country, where its enemies are possibly absent, and the truth of this has been sorrowfully enforced in some of our own colonies. The European Hyalinia cellaria, it is said, occurs literally in hundreds in the space of a few square feet near a water-fall in St. Helena; the British Cemetery at Buenos Ayres, according to a recent report, is "overloaded with Helix pomatia; our common garden snail (Helix aspersa), introduced into the Cape apparently within the memory of a living naturalist, is there astonishingly prolific; Mr. J. S. Gibbons, in 1878, mentioned that he had nowhere seen it in such plenty as near Cape

R. B. Watson, "Journ of Conch.,” vii. (1892), pp. 1-3.

1

Town; in Australia, too, as Mr. Hedley tells me, when it once obtains a footing it increases at an enormous rate, until there are far more shells to the square yard than are commonly seen in Europe; in the Botanical Gardens of Sydney, for instance, it absolutely swarms. Some hundred living specimens of Helix nemoralis, imported from England by Mr. W. G. Binney in 1857, increased with great rapidity in his garden in Burlington, New Jersey, and by 1878 the whole town was said to be full of them! At Lexington, Virginia, also, this snail, first noticed in 1886, probably introduced accidentally, appears to have multiplied at a wonderful rate; about 1889, Professor J. H. Morrison, who had transplanted the creatures to several fresh places, is said to have collected over four hundred specimens in about an hour's time within a radius of twenty-five yards.3 Slugs in similar circumstances sometimes increase with equal rapidity: in Victoria, in 1849, Mr. John Carson states, a slug found in his garden was welcomed as an old friend and carefully restored to its feeding-ground, but by the following season "matters had changed" and it was every night's work to pick up pints full"; the warm, moist autumn and mild winter of the region proved specially congenial, and the creatures became a plague, unequalled, it is said, by

ર

'J. S. Gibbons, M.B., "Quart. Journ. Conch.," i. p. 367; W. H. Rush, "Nautilus," vi. (1892), p. 81.

W. G. Binney, "Terrestrial air-breathing Mollusks," v. (1878), p. 379.

3 H. A. Pilsbry, "Nautilus," iii. (1889), pp. 51-2.