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THE DISPERSAL OF SHELLS.
FRESH-WATER SHELLS-ANOMALIES IN LOCAL
Most naturalists of recent years have agreed that "the several species of the same genus, though now inhabiting the most distant quarters of the world, must originally have proceeded from the same source, as they are descended from the same progenitor," and also, of course, that “ the individuals of the same species, though now inhabiting distant and isolated regions, must have proceeded from one spot, where their parents were first produced," and the acceptance of this view, now that it is very generally admitted that the great oceans—the most formidable of all barriers limiting the migrations of non-marine animals—have occupied the same general positions as they do now throughout all known geological periods, has naturally lent much interest and import
| Darwin, “ Origin,” ed. 6, 1890, pp. 319-20.
2 Wallace, " Island Life," 1880, p. 144 ; ed. 2, 1892, pp. 149-50 ; Darwin, “ Origin,” p. 288. For a summary of the evidence on this head, see Island Life," pp. 101-2; ed. 2, pp. 103-5.
ance to the investigation and study of means of dispersal. Obviously, we are not now authorized in explaining all the difficulties which geographical distribution presents by suggesting multiple centres of creation or former radical changes in the relative positions of land and sea.
The wide ranges and singular distributions now enjoyed by many organisms seem, in these circumstances, well-nigh inexplicable; the ranges of some genera, as everyone is aware, are almost world-wide, some species extend over immense areas, and, moreover, there are cascs in which the same or closely.allied forms occur at isolated points in remote parts of the world. Highly effectual means of dispersal of some kind or other must certainly have been in operation, for it is clear that the distribution, at least of many groups, cannot have resulted from gradual migration by ordinary modes of progression, and this is the more apparent, of course, with pre-eminently slow-moving inland animals like fresh-water and land mollusca. To such creatures evidently, as far as voluntary migration is concerned, even small arms of the sea, arid deserts, and elevated mountain-chains must be almost, cr, perhaps, absolutely, impassable ; but it has been remarked that such obstacles are not likely to have endured so long as the oceans. It will not be forgotten, of course, that the great changes of level known to have taken place “ within the period of existing organisms ” certainly remove many difficulties, and it should be borne in
| Sce" Origin," p. 317.
mind, more especially in connection with discontinuous ranges, that many types are of immense antiquity, having survived numerous changes of climate and great oscillations of level, so that the distant regions in which they are now found may, in some cases, have formed parts of former continuous and very wide ranges. It ought also to be remarked, perhaps, that some eminent biologists, as, for instance, Professor Semper, think it possible that similar faunas in distant parts of the world may sometimes have been brought about by the convergence of formerly distinct types,' and this, of course, cuts the Gordian knot much in the same way as did a formerly prevalent belief in multiple centres of special creation. Fresh-water forms are said to have been derived originally from the sea, and even now certain marine animals in all probability are gradually adapting themselves to fresh water, a process which may have been going on all through the ages, but I do not see that this much affects our present inquiry ; it should certainly be taken into account, however, whenever the occurrence at distant places of allied fresh-water forms having near relatives in the sea is considered.
Fresh-water shells, it seems, if left to their own endeavours, would generally find great difficulty in extending their range beyond the limits of their own river-basin, for the dry land obviously presents a formidable barrier to these animals, and, as Mr. Darwin remarks, each river-system with all the pools and lakes
Semper, “ Animal. Life,” Eng. transl., ed. 4, 1890, pp. 294' and 461.
in connection with it seems completely cut off from every other river-system of the same country, and still more complete, of course, is the separation between the fresh-waters of distinct islands and continents. Some of the univalves live much out of water, but, even with these, powers of voluntary migration over land must be very limited. I find, however, in Mr. Musson's manuscript, a statement that a pond-snail (Limnæa truncatula) has been seen travelling across dry fields. Bivalves seem practically incapable of migrating voluntarily beyond the limits of their own element and its immediate surroundings; some kinds, as is well known, are occasionally encountered in a living state, buried in sand, or amongst moss, &c., at distances from water, but it is unlikely that they intentionally proceed to such spots. Notwithstanding these disabilities, however, we find a wide distribution characterizing many of the creatures, both univalves and bivalves, together also with most other freshwater productions. Some species range very widely, and, as Mr. Darwin observes,
| Darwin, “ Nature,” xviii. (1878), p. 120, and see Origin," p. 343.
See, for instance, W. Thompson, “Ann. and Mag. Nat. Hist.," vi. (1841), p. 195; J. G. Jeffreys, “ British Conchology," i. (1862), p. 17 ; L. Reeve, “ Land and Fresh-water Mollusks,” 1863, p. 236; G. Roberts, “Zoologist,” (3), ix. (1885), p. 471; J. G. Milne, “ Journ. of Conch.,” vi. (1891), pp. 413-14, 418; C. T. Simpson, “Nautilus,” v. (1891), p. 16.
3 One of the pea-shells (Pisidium pusillum), it is true, is said to have been seen creeping in damp moss 6—20 paces from water ; D. Weinland, quoted in “ Zoological Record,” xiii. (1876), Moll, allied forms, which "must have proceeded from a single source," prevail throughout the world.'
Much has to be explained, also, even in local distribution. Every naturalist remembers having seen shells in puddles formed after periods of excessive rain, in pools in quarries and pits, in drinking-troughs and water-butts, in tanks on the roofs of buildings, in newly. formed reservoirs and artificial lakes, in ponds on open pastures far away from the nearest streams, &c. As the Rev. James Dalton long ago remarked, the creatures seem to possess a “mysterious faculty” of finding their way to the most unlikely habitations. Dispersal by some means must be constantly going on.
That almost every isolated cattle-pond which a farmer digs perhaps near the middle of an upland pasture should come to possess a molluscan fauna within a few years of its formation, though a matter of common observation, is certainly a surprising fact, and it is remarkable that writers of local lists when recording the existence of shells in such places have so generally passed over the circumstance without comment. If multiple birthplaces were possible it is evident, of course, that the spreading of forms from their several centres would have to be explained, for no believer in multiple centres, I imagine, would go so far as to suppose that cattle-ponds are
Origin," p. 344 ; but see Semper, “ Animal Life," p. 298. • J. Dalton, “Zoologist,” xix. (1861), 7318-9. Early in 1890 Mr. W. H. Heathcote saw a specimen of Limnæa truncatula crawling on the top of the tower of St. James' Church, Preston, ninety feet or more from the ground !