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anything ever experienced in England. Similarly among fresh-water groups the notoriously rapid multiplication and spread of the introduced zebramussel (Dreissena polymorpha) in England and certain parts of the continent of Europe may be instanced ; a statement by Sir C. Lyell that a Limnæa introduced unintentionally into Madeira by the Portuguese ran all over the island in thirty years has already been referred to. It will be borne in mind of course that molluscs, no matter how quickly they increase, can never spread with much rapidity of their own accord; obviously, migration at a "snail's pace" can hardly be rapid, and those kinds which have quickly occupied large areas must certainly have been largely helped by external
Man's influence upon a fauna, it need hardly be remarked, is not always by way of addition, for he often drives away or exterminates many aboriginal forms. The introduction of new and dominant kinds must in itself be highly prejudicial, while the disastrous results attending the destruction of forests and the breaking up of land for cultivation are but too well known. St. Helena furnishes a striking illustration. When discovered in 1501, the little island is said to have been clothed with a luxuriant forest vegetation, but the woods have been almost wholly destroyed by man and the animals introduced by him; and notwithstanding the establishment of a large number of foreign plants, its general aspect is now described as barren and
John Carson, " Garden,” xiii. (1878), p. 273.
forbidding. Much of the rich upper soil, which could only be retained on the steep slopes so long as it was protected by vegetation, has been washed away by the violent rains. As a result, we find that of the twentyseven truly indigenous species of terrestrial mollusca belonging to the island-and the fact has already been mentioned-only seven
now survive, the remainder having been exterminated, Mr. Edgar Smith remarks, “by the destruction of the primeval forests.” Foreign snails have been introduced, and are now living on the island, imported probably, perhaps as ova, with “ some of the ny introduced plants ;”? the presence of Hyalinia cellaria, for instance, has just been referred to: these, however, from a naturalist's point of view at least, form no real compensation for the loss of aboriginal forms, the destruction of an inoffensive member of a native fauna-more especially when highly peculiar-being an irreparable calamity.
Shells have been intentionally dispersed by man to a considerable extent. Both land and fresh-water kinds, as everyone is aware, have frequently been "planted,” chiefly by shell-collectors, in localities where they were previously unknown, and permanent additions to faunas, it will be admitted, have thus been made. To some persons the transplantation of foreign or local snails to spots near their residences seems to have special fascination, and a whole volume might probably be filled
Wallace, “ Island Life,” 1880, pp. 281-4; ed. 2, 1892, pp. 292-5; and see aiso Darwin's “ Journal,” ed. 2, 1845, pp. 488-9.
? See Darwin's “Journal,” pp. 489-90.
with accounts of such doings, but we are not much concerned with this kind of dispersal. Numerous unsuccessful attempts at colonization have been recorded, and these are of significance, I think, as helping us to understand how very small must be the chance of the ultimate establishment of a new colony as the result of transportal—often, no doubt, to very unsuitable spots-of a solitary specimen or a few individuals by accidental means under nature. Man often carries considerable numbers, turning them out in localities apparently wellsuited to the requirements of the particular species he wishes to acclimatize, sometimes even watching over them with care and shooting the birds which come to pick them up, yet it often happens that the creatures gradually decrease in numbers and finally die out. It must be remembered, however, that when thus carried by man they are generally put down in districts already well-stocked, and the creatures in such cases are obviously less likely to survive than those which happen to be transported by natural means to poorly stocked regions or to newly formed and uncccupied islands. The few accounts of attempts at colonization which I have noted down may possibly be worth giving. Unsuccessful attempts, it should not be forgotten, are probably less likely to be placed on record than successful ones.
The zebra mussel (Dreissena polymorpha) is said to have been planted by Mr. Stuchbury of Bristol in some of the waters near that place,' and similarly, in America,
'H. E. Strickland, " Mag. Nat. Hist.," (n. s.), ii., (1838), p. 363.
Dr. James Lewis is stated to have put down several species taken from the Mohawk River - Melantho integra, M. rufa, Goniobasis niagarensis, Somatogyrus isogonus, etc. --at the outlet of Schuyler's Lake (about eighteen miles from Mohawk), but I do not know the result of either of these experiments; an attempt made by Dr. Lewis to colonize“ Vivipara contectoides Binney (Pal. vivipara Say)” in the Mohawk River and Erie Canal, however, is said to have been eminently successful, the species having spread widely and being now “firmly established in both the canal and river," and I find it stated also that Bythinia tentaculata, planted in the Mohawk River by the same naturalist, has become very abundant, especially in the bends of the river where the water runs slowly. Planorbis corneus and B. tentaculata, planted by Miss Esmark in Norway, “increased rapidly;" Limnæa stagnalis intentionally introduced, it is said, as food for trout, into the river Avon at Christchurch, New Zealand, was said to be abundant below the Acclimatization Gardens in 1881;3 the same species, from England, thrown down in an old quarry and in deep drains at Possil Marsh near Glasgow, is known to have survived at least for some years,* and Sphærium lacustre, Paludina contecta, B. tentacu
· W. B. Marshall, “Nautilus,” v. (1892), pp. 133-4, quoting Dr. Lewis, “ American Journ. of Conch.," iv. (1868), p. 245.
2 A. F. Gray, "American Nat.,” xvii. (1883), p. 205.
3 F. W. Hutton, “ Trans. and Proc. New Zealand Inst.,” xiv. (1881), p. 157 ; C. T. Musson,“ Proc. Lin. Soc., N. S. W.” for 1890, (2), v. (1891), p. 889.
4 W. D. Roebuck, “Scot. Nat.,” 1891, p. 130.
lata, Planorbis carinatus, P. complanatus, P. corneus, L. stagnalis, and L. glabra, from localities, likely to be destroyed, in the neighbourhood of Manchester, deposited by Mr. Heathcote in quiet ponds about Farrington, were stated in 1887 to be increasing rapidly in their new homes. A good deal of colonization from pond to pond and from a distance, Mr. R. D. Darbishire states, used to be practised in the neighbourhood of Manchester; Pl. corneus, for instance, believed to have been absent from the district in 1841, “is now common in the canal at Gorton-most likely a dealer's colony;" in 1844-6, it is said, “there was a great rush to Southport' for Paludina contecta which was colonized by collectors and dealers in canals near Manchester, at Twenty Pits, Greenheys, and cotton pits, Chorlton; P. vivipara, also, at Birch, is regarded as a probable introduction.” On the other hand, L. stagnalis, put down by Mr. W. Baillie in a stream near Brora, Sutherlandshire, probably "picked by birds,” could never afterwards be found ; 3 S. corneum, B. tentaculata, and L. stagnalis, introduced into a dam near Greenock, seemed to have died out by the following year;4 and of several living specimens of L. stagnalis and Pl. corneus, placed in a small pond on
R. Standen, “ Nat.," 1887, p. 176. 2 R. D. Darbishire, cited by Mr. Standen,.“Nat.," 1887, pp. 160 and 164.
Neritina fluviatilis, B. tentaculata, and Pl.complanatus have also been planted in small numbers by Mr. Baillie in the neighbourhood of Brora.
4 T. Scott, “ Trans. Nat. Hist. Soc. Glasgow," (n. s.), i. (1887),