Water from Lime present
per gall.		 Result
River Dee, near Chester 3·00 grs. acted strongly on shells
Wrexham 4·00 grs.  „  „    „  
River Dee, near Llanderyel 0·53 grs.  „  „    „  
Trent Canal 8·33 grs. no action  „  

CHAPTER X
GEOGRAPHICAL DISTRIBUTION OF LAND AND FRESH-WATER MOLLUSCA—THE PALAEARCTIC, ORIENTAL, AND AUSTRALASIAN REGIONS

The Mollusca afford specially valuable evidence on problems of geographical distribution. This fact is largely due to their extreme susceptibility to any change in the conditions of life. Genera which are accustomed to live in a certain temperature and on certain food, cannot sustain life if the temperature falls or rises beyond certain limits, or if the required food be not forthcoming. There is therefore a marked contrast between the Mollusca of the tropics and of the temperate zones, while different regions in the same latitude, whether within or without the tropics, often show great diversity in their fauna. Every region is thus characterised by its Mollusca. The Mollusca, for instance, of Australia or of South Africa characterise those countries quite as much as do the kangaroo and the emu, the hartebeest and the ostrich; there is nothing like them anywhere else in the world. In the Greater Antilles the Mollusca stand out beyond all other forms of life as characteristic of the islands as a whole, and of each separate island in particular.

The geographical distribution of the land and fresh-water Mollusca must be considered quite apart from that of the marine Mollusca. The sea offers no such serious barriers to the spread of the latter as the land does to the spread of the former. If we were to journey to the Azores, and turn our attention to the land-snails, we should find them almost wholly peculiar, while amongst the sea-shells we should recognise many as occurring also on our southern coasts, and few that were different from those of the Mediterranean. The marine Mollusca of the Sandwich Islands, in spite of the enormous intervening distance, are not very different from those of Natal, but the land Mollusca of the two countries are as widely different as is possible to imagine.

Land Mollusca are, as has been remarked, fettered to the soil. Quadrupeds, birds, fishes, and reptiles are provided with organs of motion which enable them to overpass barriers of various kinds. Even plants, although themselves incapable of motion, may be conveyed in every direction by means of seeds, which are either wafted by the wind or adhere to the skin of animals. But the Mollusca have no such regular means of transport, and are, in a large number of instances, limited to districts of a certain character of soil, or producing certain kinds of vegetation.

The localisation, both of genera and species, occurs all over the world. The genus Achatinella, which is peculiar to the Sandwich Islands, is found there in a profusion of species. It lives in the mountain valleys which radiate from the central ridge of each island, and each valley is characterised by its own peculiar set of species. The great carnivorous Glandina is restricted to Central America and the adjacent parts of the two continents, with one or two species in Southern Europe. Bulimus proper is restricted to South America; Achatina to Africa south of the Sahara; Tornatellina to the Pacific Islands; Cochlostyla to the Philippines; Cylindrella and Bulimulus are peculiar to the New World; Buliminus, Nanina, Scarabus, and Cassidula to the Old.

Extreme cases of this restriction of habitat sometimes occur. Thus Limnaea involuta is found only in a single small mountain tarn in Ireland; Clausilia scalaris along a narrow strip of limestone in Malta; Strophia nana is confined to a few square rods on an island that is itself a mere dot in the Caribbean Sea; the genus Camptonyx occurs only in the neighbourhood of Mt. Girnar, in Gujerat; and Lantzia in moss on the top of a mountain in Bourbon.

Attempts to colonise snails in strange localities have usually resulted in failure, especially when the attempt has involved serious changes of environment. The common Cochlicella acuta of our coasts resists all endeavours to establish it beyond a certain distance from the sea. Snails brought from the Riviera and placed under almost similar conditions of climate on our own southern coasts have lived for a while, but have very rarely taken permanent root. Mr. H. W. Kew[354] has collected a good many of these attempts to acclimatise species, the general success of which seems to depend almost entirely on a restoration of the old conditions of life.

At the same time there are certain species which exhibit a curiously opposite tendency, and which seem capable of flourishing in almost any part of the world, and under the most varied surroundings. Our own common garden snail (Helix aspersa) is a striking instance of this adaptability to new conditions. It has been established, by art or by accident, in Nova Scotia, Maine, South Carolina, New Orleans, California, Mexico city, Cuba, Hayti, Cayenne, Brazil, Valparaiso, Cape Town, the Azores, St. Helena, Mauritius, Loyalty Islands, and Australia. The great Achatina fulica of East Africa has been established first in Mauritius, and from thence has been carried to the Seychelles and Calcutta. Helix lactea, a common Mediterranean species, has been carried to Teneriffe and Montevideo; Helix similaris, whose fatherland is Eastern Asia, has been transported to Mauritius, Bourbon, West Africa, West Indies, Brazil, and Australia; Ennea bicolor (Eastern Asia) to India, Bourbon, Mauritius, West Indies; Stenogyra decollata (Mediterranean basin) to South Carolina; S. Goodallii (West Indies) to British pineries; Helix Hortensis to New Jersey. Seven common English species (Limax gagates, Hyalinia cellaria, H. alliaria, Helix aspersa, H. pulchella, Pupa umbilicata) have become naturalised in St. Helena,[355] and as many as nineteen in Australia.[356]

Cases of artificial transport of this kind are readily detected; they follow the lines of trade. The snails themselves or their ova have been accidentally enclosed with plants or mould, or have adhered to packing-cases, or to hay and grass used in packing. Thus they constitute no disturbance to the general rule of the persistent localisation of species and genera, and there is little fear that the evidence which the geographical distribution of the Mollusca brings to bear upon the general problems of distribution will be confused by any intermixture of fauna naturally distinct.