254 ‘The Naturalist in Nicaragua 
but Carabide fitted for swimming, and yet that the Carabide 
are one of the groups in which the tropical members differ 
widely from the temperate ones. 
For following up this branch of inquiry the study of the 
distribution of the mollusca offers special advantages. There 
are numerous marine, fresh-water, and terrestrial species 
and genera. They are slow moving; they have not the 
means of transporting themselves great distances, like 
insects, for example, that may easily and often pass over 
arms of the sea, or fly from one country to another. Their 
shells are the commonest of fossils; and in islands such as 
Madeira and St. Helena, where we have abundant remains 
of extinct land shells, there are few, if any, of extinct animals 
of other classes or of plants. 
Taking the shells of Europe, we find a remarkable difference 
in the distribution of the land and fresh-water species. 
According to Mr. Lovell Reeve, who has specially studied 
this question, out of many hundreds of land mollusks in- 
habiting the Caucasian province at its centre in Hungary and 
Austria, only ninety extend to the British Isles, and of these 
thirty-five do not reach Scotland. Upwards of two hundred 
species of Clausilia are to be found in the centre of the pro- 
vince, and of these only four reach England, and only one 
Scotland. Out of five hundred and sixty species of Helix 
inhabiting the Caucasian province, there are but twenty-four 
in Britain. 
Whilst the distribution of the terrestrial mollusks of 
Europe is thus restricted in range, though the species are 
numerous, the fresh-water shells are few in species, but of 
wide distribution. Quoting again from Mr. Reeve:—Of the 
Lymneacea “ there are not six species, it may be safely stated, 
in all Europe, more than there are in Britain. They have no 
particular centre of creation. There is no evidence to show 
whether the alleged progenitors of our British species were 
created in Siberia, Hungary, or Thibet. There is scarcely 
any variation either in the form or number of the species in 
those remote localities. Of Planorbis scarcely more than 
fifteen species inhabit the whole Caucasian province, and we 
have eleven of them in Britain.” —‘‘ Of Physa and Lymnea, 
it is extremely doubtful whether there are any species 
throughout the province more than we have in Britain. 
