416 PHILOSOPHY OF ZOOLOGY. 



there are a few which have not been detected in Scotland^ 

 But, among the marine mollusca, the influence of climate 

 is not feh in the same degree. Living in an element, the 

 bulk and motions of which guard it equally from the ex- 

 tremes 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 decreases as we approach the poles. But as there 

 have been few cultivators of this branch of science, the geo- 

 graphical distribution of the species has been but imper- 

 fectly explored. Few parts of either England or Scot- 

 land have been surveyed by the eye of the helmintholo- 

 gist, 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 fleets 

 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 ha- 

 bituated 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 

 Marseilles, has directed his attention to this subject. He 

 found, that all these animals die immediately, if we sudden- 

 ly change their place of abode ; but that, if we gradually 

 increase the proportion of salt in the water for the one set, 

 and diminish it for the other set, we can, in general, ac- 



