struggle for life with foreign associates.' When in free intercommunication closely-allied species are not always able to invade each other's territory; two equally well fitted for their own places seem likely to be able to hold their separate habitats for almost any length of time. The Achatinellinæ of the Sandwich Islands, just referred to, so remarkable for limited specific areas, have in all probability been occasionally, though perhaps very rarely, carried by accidental means into the midst of each other's districts, but, as their distribution clearly indicates, they must generally have failed to establish themselves in the new surroundings, being unable, no doubt, to compete successfully with those already in possession. Sir C. Lyell has expressed surprise that "in the course of that vast lapse of ages which has occurred since the Newer Pliocene period" the shell-faunas of Madeira and the adjacent islet, Porto Santo, have been so little interchanged; but in all probability shells have been occasionally carried from one island to the other by natural agencies; if both are well stocked, however, and if the respective faunas are equally well able to hold their own as against the other, they are not likely to be much affected by the occasional transportal of a few individuals. Some of the characteristic Porto Santo species live in crevices of stone, and it is probable that they have often been transported to Madeira by human agency with the large quantities of stone annually 2 "Origin," p. 356. 3 "Principles," ed. 12, ii. (1875), p. 433.

1 "Origin," p. 358.

3

carried there from Porto Santo, but they have not colonized the island; on the other hand, European land-shells, believed to have been introduced by man, are said to flourish both in Madeira and Porto Santo.1

It must be admitted that neither fresh-water nor landshells are really well furnished with means for dispersal; the transportal of a species of either group over a large expanse of ocean, or to great distances on land, with subsequent establishment, must be an extremely rare and exceptional occurrence, and one which happens, perhaps, only once or twice in many hundreds of years. It can hardly be argued, therefore, that fresh-water species have been kept uniform by constant or frequent transportal. Numerous facts, it is true, apparently indicating means of dispersal for fresh-water kinds have been referred to, and these are doubtless of value as helping us to understand how it is that many have been able to wander so far from their several birth-places, but it cannot be said that they explain the wide ranges and consequent uniformity which obtain in fresh-water as against more restricted ranges and greater variety on land.

It seems evident, as Mr. Belt has remarked, that on land there has been more variation or that the varieties which have arisen in fresh-water have less frequently been preserved. Mr. Darwin has shown that in freshwater competition will have been less severe than on land, and consequently new forms will have been more slowly produced; but this consideration, taken alone, is per2 "Origin," p. 83.

2

1 "Origin," p. 357

H

haps insufficient to account for the difference in specific distribution which the terrestrial and non-marine

aquatic groups of mollusca exhibit. The problem presented is of importance, and a solution of it, which seems likely to be the true one, has occurred to Mr. Belt, who has pointed out that the variation of fresh-water species of animals and plants has probably been constantly checked by the want of continuity of lakes and rivers in time and space :

"In the great oscillations of the surface of the earth, of which geologists find so many proofs, every fresh-water area has again and again been destroyed. It is not so with the ocean-it is continuous -and as one part was elevated and laid dry, the species could retreat to another. On the great continents the land has probably never been totally submerged at any one time; it also is continuous over great areas, and as one part became uninhabitable, the land species could in most cases retreat to another. But for the inhabitants of lakes and rivers there was no retreat, and whenever the sea overflowed the land, vast numbers of fresh-water species must have been destroyed. A fresh-water fauna gave place to a marine one, and the former was annihilated so far as that area was concerned. When the land again rose from below the sea, the marine fauna was not destroyed -it simply retired farther back. There is every reason to believe that the production of species is a slow process, and if fresh-water areas have not continued as a rule through long geological periods, we can see how

variation has been constantly checked by the destruction, first in one part, then in another, of all the freshwater species; and on these places being again occupied by fresh-water they would be colonized by forms from other parts of the world. Thus species of restricted range were always exposed to destruction because their habitat was temporary and their retreat impossible, and only families of wide distribution could be preserved. Hence I believe it is that the types of fresh-water productions are few and world-wide, whilst the sea has molluscs innumerable, and the land great variety and wealth of species. This variety is in the ratio of the continuity of their habitats in time and space."

Before proceeding, in Chapter VI., to a consideration of possible means for the dispersal of land-shells, I will venture to append here a small collection of facts on their tenacity of life. As is well known, some of these creatures possess in a remarkable degree the power of retaining vitality under conditions of the most adverse kind, being able to live for long periods, even for years, without either food or moisture; and this faculty must doubtless be of the greatest use to them in their involuntary migrations, during which they must often suffer great hardships; and thus it seems likely that they are fitted for transportal by means which can never operate for frailer creatures. Even if they have had to depend almost solely on voluntary migration, as some writers seem to suppose, the ability to remain dormant

'T. Belt, "Naturalist in Nicaragua," ed. 2, 1888, pp. 334-5.

for considerable periods must have been highly useful, for we find that many kinds have learnt to evade the cold of winter by hibernation and the excessive heat and dryness of summer by æstivation, and thus they have been able to overcome climatic conditions which in other circumstances might have been fatal, and some kinds, having learnt to continue the latter process for great lengths of time, have been able to penetrate the dryest deserts. It is important, of course, to inquire also as to what extent the creatures are able to withstand the notoriously harmful effects of contact with sea-water, for on this the value of certain suggested means of trans-oceanic dispersal obviously depends; but I know only one or two facts bearing upon the point, and these can be conveniently referred to in the next chapter.

One of the most remarkable cases of long-suspended vitality I have anywhere seen recorded is given in Bingley's "Animal Biography," vol. iii. p. 574,' where some snails-on being immersed in water—are said to have recovered and crept about after an uninterrupted torpidity of more than fifteen years :

"Mr. Stuckey Simon, a merchant of Dublin, whose father, a Fellow of the Royal Society, and a lover of natural history, left to him a small collection of fossils and other curiosities, had among them the shells of some snails. About fifteen years after his father's death (in whose possession they continued many years), he by

1 As quoted by G. J[ohnston]., Loudon's " Mag. Nat. Hist.," vii. (1834), pp. 113-14.