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vertebrates. In the Helicidæ, 10 genera are confined to the New, and 7 to the Old World, 16 being common to both. In the Operculata the number of genera of restricted range is greater, — the New World having 15, the Old World 32 genera, only 8 being common to both. Of the New World genera 12 out of the 15 do not occur at all in South America; and of those of the Old World, 22 out of the 32 occur in a single region only. If we take the northern and southern division proposed by Professor Huxley (the latter comprising the Australian and Neotropical regions), we find a much less well-marked diversity. Among the Helicidæ only 4 are exclusively northern, 8 southern; while among the Operculata 22 are northern, 16 southern. The best way to compare these two kinds of primary division will be to leave out all those genera confined to a single region each, and to take account only of those characteristic of two or more of the combined regions; which will evidently show which division is the most natural one for this group. The result is as follows:

GENERA COMMON TO TWO OR MORE REGIONS IN, AND CONFINED TO, EACH

PRIMARY DIVISION OF THE EARTH.

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We find then that the northern and southern division of the globe is not at all supported by the distribution of the terrestrial molluscs. It is indeed very remarkable, that the connection so apparent in many groups between Australia and South America is so scantily indicated here. The only facts supporting it seem to be, the occurrence of Geotrochus (a sub-genus of Helix) in Brazil, as well as in the Austro-Malayan and West Pacific Islands and North Australia ; and of Bulimus in the same two parts of the globe, but peculiar sub-genera in each. But in neither case is there any affinity shown between the temperate portions of the two regions, so that we must probably trace this resemblance to some more ancient diffusion of types than that which led to the similarity of plants and insects. Still more curious is the entire

absence of genera confined to, and characteristic of Africa and India. One small sub-genus of Helix, (Rachis), and one of Achatina, (Homorus), appear to have this distribution,—a fact of but little significance when we find another sub-genus of Helix, (Hapalus), common and confined to Guinea and the Philippine Islands; and when we consider the many other cases of scattered distribution which cannot be held to indicate any real connection between the countries implicated. No genus is confined to the Palæarctic and Nearctic regions as a whole. A large number of sub-genera, many of them of considerable extent, are peculiar to one or other of these regions, but only 3 sub-genera of Helix and 2 of Pupa are common and peculiar to the two combined, and these are always such as have an Arctic range and whose distribution therefore offers no difficulty.

We find, then, that each of our six regions and almost all of our sub-regions are distinctly confirmed by the distribution of the terrestrial mollusca ; while the different combinations of them which have at various times been suggested, receive little or no support whatever. Even those remarkably isolated sub-regions, New Zealand and Madagascar, have no strictly peculiar genera of land-shells, although they both possess several peculiar subgenera; being thus inferior in isolation to some single West Indian Islands, to the Sandwich Islands, and even to the North Atlantic Islands (Canaries, Madeira, and Azores), each of which have peculiar genera. This of course, only indicates that the means by which land mollusca have been dispersed are somewhat special and peculiar. To determine in what this speciality consists we must consider some of the features of the specific distribution of this group.

The range of genera, and even of sub-genera is, as we have seen, often wide and erratic, but as a general rule the species have

a very restricted area. Hardly a small island on the globe but has some land-shells peculiar to it. Juan Fernandez has 20 species, all peculiar. Madeira and Porto Santo have 109 peculiar species out of a total of 134. Every little valley, plain, or hill-top, in the Sandwich Islands, though only a few square miles in extent, has its

peculiar species of Achatinella. Another striking feature of the distribution of land molluscs, is the richness of islands as compared with continents. The Philippines contain more species than all India ; and those of the Antilles according to Mr. Bland almost exactly equal the numbers found in the entire American continent from Greenland to Patagonia. Taking the whole world, it appears that many more species of land-shells are found in the islands than on the continents of the globe, a peculiarity that obtains in no other extensive group of animals.

Looking at these facts it seems probable, that the air-breathing molluscs have been chiefly distributed by air- or water-carriage, rather than by voluntary dispersal on the land. Even seas and oceans have not formed impassable barriers to their diffusion; whereas they only spread on dry land with excessive slowness and difficulty. The exact mode in which their diffusion is effected is not known, and it may depend on rare and exceptional circumstances; but it seems likely to occur in two ways. Snails frequently conceal themselves in crevices of trees or under bark, or attach themselves to stems or foliage, and either by their operculum or mucous diaphragm, are able to protect themselves from the injurious effects of salt water for long periods. They might therefore, under favourable conditions, be drifted across arms of the sea or from island to island; while wherever there are large rivers and occasional floods, they would by similar means be widely scattered over land areas. Another possible mode of distribution is by means of storms and hurricanes, which would carry the smaller species for long distances, and might occasionally transport the eggs of the larger forms. Aquatic birds might occasionally get both shells and eggs attached to their feet or their plumage, and convey them across a wide extent of sea. But whether these, or some other unknown agency has acted, the facts of distribution clearly imply that some means of transport over water is, and has been, the chief agent in the distribution of these animals; but that its action is very rare or intermittent, so that its effects are hardly perceptible in the distribution of single species.

Another important factor in enabling us to account for the

distribution of these animals is the geological antiquity of the group, and the amount of change exhibited in time, by species and genera. Now we find that most of the genera of land-shells range back to the Eocene period, while those inhabiting fresh water are found almost unchanged in the Wealden. In North America a species of Pupa and one of Zonites, have been discovered in the coal measures, along with Labyrinthodonts; and this fact seems to imply, that many more terrestrial molluscs would be discovered, if fresh-water deposits, made under favourable conditions, were more frequently met with in the older rocks. If then the existing groups of land-molluscs are of such vast antiquity, and possess some means, however rarely occurring, of crossing seas and oceans, we need not wonder at the wide and erratic distribution now presented by so many of the groups; and we must not expect them to conform very closely to those regions which limit the range of animals of higher organization and less antiquity.

The total number of species of pulmoniferous mollusca is about 7,000, according to the estimate of Mr. Woodward, brought down to 1868 by Mr. Tate. But this number would be largely increased if the estimates of specialists were taken. Mr. Woodward for example, gives 760 as the number of species in the West Indian Islands; whereas Mr. Thomas Bland, who has made the shells of these islands a special study, considers that there were 1,340 species in 1866. So, the land-shells of the Sandwich Islands are given at 267; but Mr. Gulick has added 120 species of Achatinellidæ, bringing the numbers up to nearly 400,- but no doubt several of these are so closely related that may chologists would class them as varieties. The land-shell fauna of the Antilles is undoubtedly the most remarkable in the world, and it has been made the subject of much interesting discussion by Mr. Bland and others. This fauna differs from that of all other parts of the globe in the proportions of the operculate to the inoperculate shells. The Operculata of the globe are about one-seventh, the Inoperculata about six-sevenths of the whole ; and some general approximation to this proportion (or a much sinaller one) exists in almost all the continents, islands, and

archipelagoes. In the Philippines, for example, the proportion of the Operculata is a little more than one-seventh; in the Mauritius, between one-third and one-fourth ; in Madeira, onefourteenth ; in the whole American continent about one-eighth; but when we come to the Antilles we find them to amount to nearly five-sixths, about half the Operculata of the globe being found there!

Mr. Bland endeavours to ascertain the source of some of the chief genera found in the West Indian Islands, on the principle that “ each genus has had its origin where the greatest number of species is found;" and then proceeds to determine that some have had an African, some an Asiatic, and some an American origin, while others are truly indigenous. But we fear there is no such simple way of arriving at so important a result; and in the case of groups of extreme antiquity like the genera of mollusca, it would seem quite as possible that the origin of a genus is generally not where the greatest number of species are now found. For during the repeated changes of physical conditions that have everywhere occurred since the Eocene period (to go no further back) every genus must have made extensive migrations, and have often become largely developed in some other district than that in which it first appeared. As a proof of this, we not unfrequently find fossil shells where the species and even the genus now no longer exists; as Auricula, found fossil in Europe, but only living in the Malay and Pacific Islands; Anastoma and Megaspira, now peculiar to Brazil, but fossil in the Eocene of France; and Proserpina of the West Indies, found in the Eocene formation of the Isle of Wight. The only means by which the origin of a genus can satisfactorily be arrived at, is by tracing back its fossil remains step by step to an earlier form; and this we have at present no means of doing in the case of the land-shells. Taking existing species as our guide we should certainly have imagined that the genus Equus originated in Africa or Central Asia ; but recent discoveries of numerous extinct species and of less specialized forms of the same type, seem to indicate that it originated in North America, and that the whole tribe of "horses" may be, for anything we yet know

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