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 flcetz 

 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, 



