tion."1 Some Limnææ, &c., we must remember in connection with these facts, are not altogether incapable of migration on land, for they are nearly amphibious; Limnæa peregra, for instance, Mr. Jeffreys states, as its name imports, is "fond of wandering and seeing a little of the world," and Limnæa truncatula, it is even said, is "more frequently met with out of the water than in it."

1 "Life, Letters, and Journals of Sir C. Lyell, Bart.", ii. (1881), 209, 212, in letters dated 1856.

2 "British Conchology," i. (1862), 107, 116.

CHAPTER II.

MEANS OF DISPERSAL.

THE manner in which fresh-water shells are distributed over the globe, the wide ranges enjoyed by many, and their local diffusion in isolated waters, as we have seen, clearly imply the existence of means of dispersal. The creatures must certainly have been carried from pool to pool, from river-basin to river-basin, and from mainland to island, even, indeed, to some of the most isolated archipelagoes of the open ocean.

Objections to suggested means have sometimes been raised on the ground that the creatures would probably be unable to withstand the exposure to which they would be subjected, but, as is pretty generally known, many kinds, at least, are able to live for a time out of water, and under the most adverse conditions. Of a number of pond-snails (Limnæa truncatula), for instance, placed by Professor A. P. Thomas in an open vessel, in a dry laboratory, where the sunshine fell upon them for an hour or so daily, rather more than fifty per cent. survived for twenty-six days, and some few were alive after more than six weeks. A specimen of Paludina 1 A. P. Thomas, "Quart. Journ. Micro. Sci.," (n.s.), xxiii. (1883) 131.

2

vivipara, an operculate water-snail, as Mr. R. M. Christy states, accidentally ejected from an aquarium and allowed to remain for more than three weeks on the ground in a field, on being restored to its fellows seemed to have taken no harm. Some Ampullaria, placed in a drawer by Mr. Laidlay, are even said to have survived, in the warm climate of Calcutta, for five years! A freshwater mussel (Anodonta) once arrived alive in France after having been wrapped up in dry paper for eight months during its voyage from Cochin-China,3 and an Australian Unio, resembling our "swollen fresh-water mussel" (Unio tumidus), having already survived in a dry drawer for 231 days, packed up (after being tested in water) and forwarded to England, reached Southampton in a living state 498 days after its capture, and was subsequently "restored to its element, with full vital powers," in the care of Dr. Baird, of the British Museum." This great tenacity of life, common to many kinds, of which quite a number of instances might be given," must certainly have largely facilitated dispersal.

1 R. M. Christy, "Zoologist," (3), v. (1881), 181. 2 Woodward's "Manual," ed. 4, rep., 1890, p. 14.

3 "Journ. de Conch.," xxiii. (1875), 81-4, as quoted in "Quart. Journ. of Conch.," i. p. 78, and "Zoological Record," xii. (1875), 136.

4 Probably, according to Mr. Musson, U. ambiguus or U. australis.

5

J. S. Gaskoin," Proc. Zool. Soc.," 1850, pp. 243-4, and see also Woodward's "Manual," ed. 4, rep. 1890, pp. 13-14.

• See, for example, J. L. Hawkins, on Limnæa stagnalis, "Science Gossip," xvii. (1881), 23; Jeffreys, on Sphærium lacustre, "British Conchology,” i. (1862), 11-12; Baker Hudson, on Sphæ

But we must inquire as to the means by which the creatures have been diffused. The problem, everyone is aware, is an old one, and has been answered or partially answered by many writers; Sir C. Lyell attended to it, as also did Mr. Darwin, and many naturalists have followed in the footsteps of these distinguished authors.

In early life, collecting in the fresh waters of Brazil, Mr. Darwin noted with surprise the similarity of the fresh-water insects, shells, &c., and the dissimilarity of the surrounding terrestrial forms of life, as compared with those of this country; and questions as to the diffusion of the former seem to have been more or less prominently before his mind during many years; a letter on means of dispersal of bivalves, one of his latest published writings, appeared in "Nature" only thirteen days before his death. In the "Origin" he expressed the belief that the wide ranging power of fresh-water productions could be explained, in most cases, "by their having become fitted, in a manner highly useful to them, for short and frequent migrations from pond to pond, or from stream to stream, within their own countries," and liability to wide dispersal, he remarked, “would follow from this capacity as an almost necessary consequence." The way in which such migrations are

1

rium corneum, "Science Gossip," xx. (1884), 91; C. T. Simpson, on Unio, &c., "Conchologists' Exchange," ii. (1887), 50; E. B. Sanger, on an Australian fresh-water mussel, "American Nat.," xvii. (1883), 1184-5; and J. E. Gray, on Dreissena polymorpha, "Annals of Philosophy," ix. (1825), 139.

1 "Origin," p. 343.

probably effected was discussed in some detail, and diversified and curious means of dispersal were shown to be in operation; as regards molluscs, facts were given suggesting transportal, possibly over the sea, by aquatic birds and insects; and subsequently, in 1882, when publishing several additional facts indicating dispersal by animal agencies, Darwin demurred to the belief, which had been somewhat doubtfully expressed by Mr. Jeffreys, that the diffusion of fresh-water shells "had a different and very remote origin, and that it took place before the present distribution of land and water." 2

Much diffusion, of course, as Darwin admitted, must be attributed to changes in the level of the land," within the recent period, causing rivers to flow into each other." 3 Streams belonging to distinct systems, Mr. Wallace has remarked, often approach and might be made to change their course from one to the other basin by very slight alterations of level. During the glacial epoch, ice, blocking up valleys, compelled streams to flow over watersheds to find an outlet.*

The carrying powers of floods, as well as of rivers in their ordinary action, are doubtless very great, and must have largely influenced distribution. Professor Ralph Tate, indeed, long ago expressed the opinion that the

Darwin, "Nature," xviii. (1878), 120-1; xxv. (1882), 529-30; Jeffreys," British Conchology," i. (1862), lxxx.

3 66 Origin," p. 344.

4 Wallace, "Island Life," (1880), pp. 74 and 324; ed. 2, pp. 76 and 344.