270 On the Distribution of Marine Species. 



The temperature of the waters near southern England in 

 summer is 62°, and near the Shetlands 55°, or less. Conse- 

 quently the surface summer temperature of the British Chan- 

 nel is not found in the iEgean at a less depth than thirty- 

 five fathoms, and the surface summer temperature of the 

 Shetlands is the temperature at one to three hundred fathoms 

 in the iEgean ; and still species that range to a depth of one 

 hundred fathoms about northern Scotland, are found within 

 thirty fathoms of the surface in the iEgean ; that is, where the 

 summer temperature is 74° or more. Such facts shew the har- 

 diness of the species in enduring great ranges in temperature. 

 We must therefore conclude that it is not temperature alone, 

 or mainly, which determines the depth to which species may 

 live. It exerts an influence, and species fitted for cold waters 

 may be found in the deeper seas where such waters occur. 

 But the limit of descent depends on other influences. 



Looking at this table in another way, we see, as recognized 

 by Professor Forbes, that species which occur at or near the 

 surface in northern Scotland, are generally met with only at 

 greater depths in the Mediterranean ; that is, the minimum 

 depth is less in the former case than in the latter. Thus Cor- 

 bula nucleus has for its minimum depth in the Mediterranean 

 six fathoms, and in the northern regions three fathoms. 

 Psammobia ferroensis has ten fathoms for the former, and 

 three for the latter. Other examples will be found in the 

 above table, sufficient to illustrate the principle, although 

 many exceptions exist. Thus species that have a range of 

 one hundred fathoms beyond Scotland may have the same in 

 the Mediterranean, except that in many cases they do not reach 

 as near the surface, where the waters are warm. 



The Crustacea of the same seas illustrate this subject in a 

 similar way ; but the observations upon them have been 

 made with less thoroughness, and we have therefore confined 

 our discussions to Molluscs. — {American Journal of Science 

 and Arts, vol. xv., 2d series, No. 44, p. 204.) 



