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Turton, while he was writing his book on British land-shells, the present writer learnt more of the art of observing than he would have learnt in three years' desultory hunting on his own account; and he has often regretted that no naturalist has established shore-lectures at some watering-place, like those up hill and down dale field-lectures which, in pleasant bygone Cambridge days, Professor Sedgwick used to give to young geologists, and Professor Henslow to young botanists.
It is, however, not at all probable that land-shells have often been thus transported; the feet of birds offer a more probable method. The most striking and important fact for us is the affinity of the species which inhabit islands to those of the nearest mainland, without being actually the same. Numerous instances could be given.
The bones, he suggested, may often have been rolled in the beds of such streams before they reached their underground destination. To the same agency the introduction of many land-shells dispersed through the cave-mud was ascribed, such as Helix nemoralis, H. lapicida, H. pomatia, and others of living species.
This time, the Resin-bee is settled in Helix algira, the most remarkable of our land-shells because of the width and regularity of its spiral, which is copied from that of the Ammonites. This magnificent nest, a perfect specimen of both the Snail's work and the Bee's, deserves description before any other. At this inconsiderable depth, a partition is clearly seen.
In the pools of water I found but few aquatic beetles, and not any fresh-water shells: Succinea at first appears an exception; but here it must be called a terrestrial shell, for it lives on the damp herbage far from water. Land-shells could be procured only in the same alpine situations with the beetles.
The superficial deposit, moreover, is disturbed almost everywhere by agricultural labours, and even were this not the case, the action of worms, insects, and the roots of plants would suffice to confound together the deposits of two successive years. All the remains of organic bodies, such as land-shells, and the bones of quadrupeds, found during the excavations belonged to living species.
It occurred to me that land-shells, when hybernating and having a membranous diaphragm over the mouth of the shell, might be floated in chinks of drifted timber across moderately wide arms of the sea.
As they are seen only very early in the morning, when the ground is slightly damp with dew, the Guasos believe that they are bred from it. I have observed in other places that extremely dry and sterile districts, where the soil is calcareous, are extraordinarily favourable to land-shells.
It occurred to me that land-shells, when hybernating and having a membranous diaphragm over the mouth of the shell, might be floated in chinks of drifted timber across moderately wide arms of the sea. And I find that several species in this state withstand uninjured an immersion in sea-water during seven days.
Madeira, again, is inhabited by a wonderful number of peculiar land-shells, whereas not one species of sea-shell is confined to its shores: now, though we do not know how seashells are dispersed, yet we can see that their eggs or larvae, perhaps attached to seaweed or floating timber, or to the feet of wading-birds, might be transported far more easily than land-shells, across three or four hundred miles of open sea.
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