MOLLUSCA. 21 
greatly limiting their geographical range. In proof of this, 
it may be stated, that the south of France possesses several 
species not to be found in England, whilst in England, there 
are a few which have not been detected in Scotland. But, 
among the marine mollusca, the influence of climate is not 
felt in the same degree. Living in an element, the bulk 
and motions of which guard it equally from the extremes of 
heat or cold, these animals, like the sea-weeds, have a very 
extensive latitudinal and longitudinal range. Thus, some 
are common to Greenland and the Mediterranean, others to 
Britain and. the West Indies. The mollusca of the tropical 
seas, however, differ widely as a whole from those of the 
temperate regions. Some of the forms appear to be peculiar 
to warm regions, and, in general, the intensity of colour de- 
creases as we approach the poles. But as there have been 
few cultivators of this branch of science, the geographical 
distribution of the species has been but imperfectly explor- 
ed. Few parts of either England or Scotland have been 
surveyed by the eye of the helminthologist, so that many 
species, the range of which is considered as limited, may 
soon be found to be extensive. 
If the observations are few and imperfect, which have 
been made on the influence of temperature, in regulating 
the physical distribution of mollusca, we are still in greater 
ignorance with regard to the power of habit. In the floetz 
rocks, the relics of marine and fluviatile mollusca are found 
mixed in the same bed. This circumstance gave rise to the 
inquiry, how far the mollusca of fresh water can be habitu- 
ated to sea-water, and vice versa. In the account of the 
proceedings of the National Institute of France, for the 
year 1816, it is stated, that M. Beuchant, professor at Mar- 
seilles, has directed his attention to this subject. He found, 
