116 EXPLANATIONS. 



of the sea, those which can survive the change 

 assume considerably different characters. In a 

 fresh-water tertiary formation of the island of Cos, 

 Professor Edward Forbes and Lieutenant Spratt 

 foimd various fresh-water molluscan shells — palu- 

 dina, neretina, melanopsis, etc. — which had passed 

 through surprising modifications in the coiu-se of 

 three successive groups of deposits, supposed to 

 have been marked by increasing influxes of sea- 

 water. " The lowermost species of each genus 

 were smooth, those of the centre partially plicated, 

 and those of the upper part strongly and regularly 

 ribbed."* This was apparently a retrogression to 

 marine types. The differences in the thi-ee cases 

 were greater than those which naturalists usually 

 consider as grounds of specific distinction. 



Surely there are here ample evidences of spe- 

 cies, or what are usually regarded as such, being 

 variable under changed conditions. It will be 

 said, these changes are all mere variations of spe- 

 cific forms, and the facts do nothing but show 

 that that has been called species which is only 

 variety. But where is this to have its limits } If 

 the cabbage and sea-plant are to be now regarded 

 as one species, it seems to me that we have to go 



* Report of Proceedings of the British Association, 1845. — Lite- 

 rary Gazette. 



