February 15, 1894] 



NA TURE 



;6i 



scriptive methods that are available. It is, moreover, 

 essentially incompetent, and therefore unsatisfactory. 



But if we pass over such considerations as these, funda- 

 mental as they are from some points of view, recognising 

 that in practical life we often count it a saving of time to 

 exhaust the logical consequences of a belief, before pain- 

 fully verifying the grounds of that belief and recognising 

 that in a new subject it is always the most elementary 

 method that furnishes the easiest introduction, we shall 

 find very much for which to value the book before us— 

 beside the excellent diagrams, index, printing, &c. 



Xote I. — The result of carrying out in detail the work 

 mentioned at the end of § 194 seems worth introducing 

 into a new edition. The result is partly given in Math. 

 Tripos. Part II. 1S92. It seems a pity, too, that the 

 expressions for the Jacobian Z{u) in terms of the 

 Weierstrassian ^{ti) are not given. 



Note 2.— The example 15, p. 351 (though taken from 

 Math. Tripos, Part II.), is wrong. The result should be 



— .-. = - (-fi + -Vo + -^3) = — 



{.x\ + x.^ + X.,) 



\\\\exp,y = inx-TC is the final position of the line. 

 Note 3. — The example 2 (i.), p. 140, is misprinted. 



H. F. Baker. 



THE DISPERSAL OF SHELLS. 

 The Dispersal of Shells : an Inquiry into the Means 

 of Dispersal possessed by Fresh-water and Land 

 Mollusca. By Harry Wallis Kew, F.Z.S. With a 

 Preface by Alfred Russel Wallace, LL.D., F.R.S. 

 With Illustrations. Pp. xi. 291. International Scientific 

 Series, \o\. Ixxv. 8vo. (London : Kegan Paul, Trench, 

 Triibner and Co., 1893.) 



IT is strange that we have had to wait so long for a 

 manual on dispersal. Many books have been 

 written on the geographical distribution of animals and 

 plants ; and islands and even continents have been raised 

 or lowered to account for the strange anomalies. Yet 

 comparatively little attention has been paid to a study 

 that must be undertaken before we are qualified to ex- 

 press an opinion on geographical distribution. Darwin and 

 Lyell, however, thoroughly recognised the importance of 

 the subject, and the former made many experiments on 

 the vitality of seeds under trying circumstances — such as 

 being immersed in sea-water, or eaten by birds. Direct 

 observation of the species in transit, under natural 

 conditions, has been less attended to, except in the case 

 of flying animals and of certain plants. The cause of 

 this neglect is easy to understand : dispersal, in the 

 groups that are not specially modified to assist the 

 process, is mainly the result of the accumulation of rare 

 accidents, such as would only occasionally be noticed by 

 some naturalist engaged in quite different observations. 

 It is useless to go into the field on purpose to watch the 

 dispersal of snails ; the entomologist, ornithologist, 

 fisherman, or sportsman may once in a season obtain a 

 direct observation, and it is to such observers that we 

 must principally trust. 



In certain respects the land and fresh-water mollusca 

 are peculiarly valuable for the study of geographical 



I distribution ; they are essentially sedentary animals ; 



j some of them can float, but scarcely any except 

 NO. T268, VOL. 49] 



Dreissena have an active free-swimming larval stage. 

 Few of the species are specialised for dispersal ; though 

 we do not think that there is such a complete absence of 

 specialisation as would at first appear. The study of 

 the dispersal of the mollusca becomes, under these 

 circumstances, of great importance to the naturalist ; for 

 if snails or their eggs can cross rivers and straits, it is 

 probable that other sedentary groups can do so also. 



The system on which Mr. Kew has worked is to collect 

 all the facts relating to the dispersal of land and fresh- 

 water mollusca, giving the authority for each statement. 

 He has thus gathered into one small volume an enormous 

 amount of information, much of which will be quite new 

 to naturalists. Beginning with the fresh-water shells, he 

 treats first of the anomalies in their local distribution, 

 such as their occurrence in perfectly isolated ponds. 

 Then follow chapters dealing with the means of dis- 

 persal ; and it is surprising how varied these are. Not 

 only are the animals transported down stream on various 

 floating objects, but the author can quote an actual 

 instance in which a number of fresh-water mollusca 

 {Atiodon) were carried by a whirlwind and fell with the 

 rain. Another interesting case of transportation over 

 dry land is that mentioned by Canon Tristram, who 

 found the eggs of some mollusc, probably Sicccinea 

 attached to the foot of a passing mallard shot by him in 

 the Sahara, a hundred miles from water. A few instances 

 are noted in which birds on the wing have been shot 

 with bivalves adhering to their toes ; but there seems to 

 be no recorded case of the occurrence of molluscs or 

 their eggs in the bits of water-weed that so often catch 

 on the feet of aquatic birds. It is probable that this 

 means of transport is common ; but being less striking 

 than the other modes, it has not yet been observed. 

 Insects also lend their aid, and a water-beetle {Dytiscus 

 marginalis) has twice been captured on the wing with 

 Sphceriiim attached to its legs ; another specimen 

 was caught with Ancylus attached to its wing-case. 

 Various other aquatic insects have often been found 

 with mollusca attached to them, though they were not 

 actually caught on the wing. 



As regards the land-shells, there is a singular dearth 

 of direct evidence. Mr. Kew is able to mention various 

 ways in which they may have been transported ; but the 

 only cases in which the process has actually been ob- 

 served were some live Helix caperata found in a wood- 

 pigeon three days after it had been shot, and an opercu- 

 lated land snail which had caught the foot of a bumble- 

 bee, and was being dragged along. We cannot help 

 thinking, however, that the dispersal of land-shells is a 

 much rarer process than the carrying of fresh-water 

 species. An isolated dew-pond after an existence of 

 ten years will generally yield several species of fresh- 

 water mollusca, and a mediaeval fish-pond has c[uite a 

 large fauna. A church or castle built of limestone, 

 but surrounded by non-calcareous desert, is, for a large 

 group of land snails, the equivalent of an isolated pond ; 

 but it is only on very old buildings that one finds colonies 

 of the special limestone species. We have never come 

 across an isolated colony of this sort on a building less 

 than a hundred years old, and have never noticed more 

 than two or three species on a single ruin under such 

 circumstances. 



