the species entirely disappeared and was never again noticed in the locality.[106] Writing to the Zoological Society of London from New Caledonia, Mr. E. L. Layard remarks:[107] “The West Indian species _Stenogyra octona_ has suddenly turned up here in thousands; how introduced, none can tell. They are on a coffee estate at Kanala on the east coast. I have made inquiries, and cannot find that the planter ever had seed coffee from the West Indies. All he planted came from Bombay, and it would be interesting to find out whether the species has appeared there also.” Sometimes a very small event is sufficient to disturb the natural equilibrium of a locality, and to become the cause either of the introduction or of the destruction of a species. In 1883 a colony of _Helix sericea_ occupied a portion of a hedge bottom twenty yards long near Newark. It scarcely occurred outside this limit, but within it was very plentiful, living in company with _H. nemoralis_, _H. hortensis_, _H. hispida_, _H. rotundata_, _Hyalinia cellaria_ and _Hy. nitidula_, and _Cochlicopa lubrica_. In 1888 the hedge was well trimmed, but the bottom was not touched, and the next year a long and careful search was required to find even six specimens of the _sericea_.[108] =Showers of Shells.=--_Helix virgata_, _H. caperata_, and _Cochlicella acuta_ sometimes occur on downs near our sea-coasts in such extraordinary profusion, that their sudden appearance out of their hiding-places at the roots of the herbage after a shower of rain has led to the belief, amongst credulous people, that they have actually descended with the rain. There seems, however, no reason to doubt that Mollusca may be caught up by whirlwinds into the air and subsequently deposited at some considerable distance from their original habitat, in the same way as frogs and fishes. A very recent instance of such a phenomenon occurred[109] at Paderborn, in Westphalia, where, on 9th August 1892, a yellowish cloud suddenly attracted attention from its colour and the rapidity of its motion. In a few moments it burst, with thunder and a torrential rain, and immediately afterwards the pavements were found to be covered with numbers of _Anodonta anatina_, all of which had the shell broken by the violence of the fall. It was clearly established that the shells could not have been washed into the streets from any adjacent river or pond, and their true origin was probably indicated when it was found that the funnel-shaped cloud which burst over the town had passed across the one piece of water near Paderborn, which was known to contain the _Anodonta_ in abundance.