DECLINE OF SPECIES. 135 



descendants, if they have any, will be of still feebler 

 character ; and, should the modifying cause continue 

 to increase, the species will then probably cease to exist. 

 The sudden and complete influx of fresh water to a 

 basin previously covered by the sea would instantane- 

 ously kill all its testaceous animals. But were the 

 change gradual, these same animals and their descend- 

 ants would exist in water considerably less salt ; though , 

 they would probably cease to propagate before the lake 

 had become wholly fresh. A curious instance of gra- 

 dual change in a fossil marine species from the influx of 

 fresh water, was observed by Professor E. Forbes* in the 

 island of Cos. In this case the change of circumstance 

 was clearly marked, through the several beds in which 

 the shell occurred, till the 

 species altogether ceased. 

 These forms or varieties, 

 depending on the influx 

 of fresh water, are shown 

 in the annexed figure. 

 I have noticed similar 

 change in the character 



of the common Littorina rudis of our own coasts, whose 

 shell, when the animal occupies its proper habitat, be- 

 tween tide-marks, is thick and strong, with shallow 

 grooves between the spires. But this species sometimes 

 climbs up rocks of considerable height, and remains 

 upon them, trusting to the washing of the spray for its 

 nourishment. The specimens I allude to were found 

 among the crevices of a sea-cliff on the West of Ireland, 

 * " Travels in Lycia," 2 vols. 8vo., 1847. 



