4'4 



the water and pass there the first and often the longest part of 

 their existence, though they leave that element when arrived at 

 maturity. 



The MoUusca, too. must not be passed over, though by far the 

 gi-eater number of species in this large group are marine. A few 

 are found in fresh water ; more upon land. And there is one 

 circumstance connected with these last which I think deserves 

 mention, as showing that the distinguishing characters of animals 

 are affected by the medium in which they live, and so far of 

 importance in respect to the Darwinian theory, viz., that the land 

 species are as a whole less variable than the aquatic* Of land 

 shells in this country we have about 65 species ; and there are 

 scarcely more than 10 of these which present any difficulty in the 

 determination of the specific characters, the most variable being 

 the Succinece, which though not actually aquatic, are always found 

 in the neighbourhood of water, and often upon aquatic plants. 

 Among the true aquatic shells, less numerous than the land, a large 

 proportion are variable ; and in many instances there is difference 

 of opinion among conchologists as to what are to be accounted 

 species and what are varieties, the variation being especially 

 noticeable in the fresh-water bivalves, one-half at least of which 

 have been split into two or more species according to the views 

 of those who have made a study of them. It is also observable 

 how, even in the case of those aquatic shells about which most 

 are agreed, we find the species varying according to the nature of 

 the water in which they live, rapid streams generally having the 

 effect of making the shell smaller, and the valves of bivalves 

 thicker and stronger, and often more rugged outwardly ; the same 

 shells in stagnant waters growing to a much larger size, with the 

 valves thinner and of a more delicate texture. 



* See, however, an important article in "Nature" (vol. vi.,p. 222) on the 

 variation of land-shells, as illustrated in the Achatinellidm, in one of the 

 Sandwich Islands ; where a remarkable number of species and varieties 

 of the group in question are found within a very limited area, being much 

 influenced in character by the circumstances of their geographical dis- 

 tribution. — Though relating to a single family of shells in a distant region of 

 the globe, the statement is suggestive of what might come of a more close 

 inquiry into any other group similarly studied in any other locaHty. 



