306 Notes on the Indian terrestrial Gasteropoda. | No. 4, 
wedged in a cleft stick or cemented to a bough, I fully admit, but not 
if the animal was exposed to the moist sea air, not to say immersed 
in either fresh or salt-water, as such a condition would certainly pro- 
voke its dormant vitality and lead to its destruction ; and it must be 
remembered, that the period when shells are most liable to be car- 
ried down by streams, is just the time when they are most active and 
unlikely to be hermetically fastened to their hot-weather roosting- 
places : the method of diffusion therefore of shells on floating wood, I 
must, though of course opinions will differ, regard as strained and 
improbable even as an exceptional case. 
Equally difficult is it to realize the dispersion by the accidental 
method of the freshwater shells of India, enjoying as they do a far 
wider range than the terrestrial races, though they must be less fit- 
ted than any land shell for transmission across a submerged country. 
Birmah for example possesses its own peculiar Uniones, but many Indian 
freshwater shells (including Unio ceruleus and U. marginalis) range 
our Birmah also, though it will hardly be contended that these shells 
navigated the salt seas on sticks, or ploughed their way inchmeal over 
every water-shed between the Indus and Irawadi; neither is it easy 
to say, if such means were really subservient to the diffusion of the 
wide spread species, how the peculiar or local species which are the 
most numerous, did not come to extend their range in like manner. 
Indeed it seems to me an inevitable conclusion, if we admit the 
potency of accidental transmission to extend the range of a species, 
that no such thing as a local fauna, possessing a special facies and 
circumscribed limit, could exist, or at the most that such a limitation 
of species among the mollusca would be the exception and not the 
rule, which our knowledge of the distribution of Indian mollusca, (to 
confine myself to my topic,) emphatically disproves. With whatever 
theory we associate the fact, few will deny that all animals have a 
definite range in space, no less than in time, and which is not the less 
a reality, because it is capable of extension or mutation under the in- 
fluence of physical conditions, just as the normal life of an animal 
may be prolonged or curtailed by the conditions by which it is sur- 
rounded; and as some genera enjoy a far larger range in time than 
others in the vista of the past, so some genera and species are far more 
widely spread as regards space than others, whose restricted areas 
are swallowed up as it were in the domain of their numerically in- 
ferior allies. 
