1863. ] Notes on the Indian terrestrial Gasteropoda, 309 
To account for the diffusion of the descendants of a single pair, two 
modes are usually suggested as adequate, viz., voluntary migration of 
individuals, and their fortuitous transport on floating logs or lesser 
vehicles over the watery waste. Now the spontaneous migration of 
animals can only be brought about by the operation of two causes,— 
a natural instinct such as regulates the movements of migratory birds» 
the operation of which we may safely dismiss from our present cal- 
culations, or the necessity for seeking more abundant supplies of food, 
or of avoiding unsuitable physical conditions. In the case, however, 
of the invertebrata we are considering, the second cause can have but 
little more effect than the first in stimulating their movements, as we 
know of no migratory hosts of snails, compelled by dearth of food to 
shift their quarters, or of any who adopt any other mode than torpidi- 
ty to escape the prejudicial effects of either extreme heat or cold: 
representing therefore the results of migration as a, we may safely 
conclude that «0. ‘There still remains the very varied, and as it 
is usually deemed, efficacious mode of accidental transport by winds 
and waves through the agency of floating wood and the like. Cer- 
tainly a complete list of all the terrestrial shells which have trusted 
their lives to floating wood since the days of Adam, would, maugre 
its small size, prove a formidable difficulty for me to dispose of, and 
I am prepared to admit that many cases of dispersion by accidental 
agencies may have occurred, and to give the full weight to this cause, 
which I think it is fairly entitled to ; but it is not to explain an occa- 
sional or exceptional case, that the power of floating wood is invoked, 
but to account for the general diffusion of individuals throughout the 
area of the species, and this, as I have before remarked, seems utterly 
disproved by great numerical preponderance of species enjoying very 
restricted areas, (most strikingly so among the operculata,) a few only 
overrunning the boundary of the province in which they occur, and 
of these which do so, some extend their range far beyond any strictly 
Indian Province. To illustrate this more forcibly, I give a tabular 
statement, exhibiting the distribution of the two most important 
families of terrestrial mollusks in India, viz., the Helicide and Cy- 
clostomide ; prefacing it with a few words respecting the provinces, 
into which I conceive India and the adjacent countries may be divid- 
ed. The provinces as now proposed depend in a great measure on 
physical demarcation, and are therefore to some extent natural divi- 
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