THE LOCAL EXTINCTION OF MOr.LUSCS. 93 



therefore, the food, &c., does affect the distribution and the dis- 

 tribution does not affect the perpetuation or loss of a variety, we 

 have here an important factor, of which probably but very little 

 is known. 



NOTE. — Since writing the above I have obtained additional 

 evidence as to the local distribution of Helix vivgata. I have 

 found it in great numbers at another point about three miles west 

 of Felstead. As, however, there is not a scrap of evidence that I 

 am aware of to show that it has ever lived at Felstead, I must 

 still regard it as an incoming species. 



Many other species occur in patches, as it were, at distant 

 intervals, but they are always, so far as I know, joined by here 

 and there an individual. This is certainly not the case with Helix 

 vivgata, and on such grounds, and in the entire absence of fossils, 

 I must hold it to be a new comer. — J. F. 



[Mr. French ended by giving some observations on sug- 

 gested causes of variation in the Mollusca. But speculations on 

 this most difficuU subject, in the absence of long-continued expe- 

 rimental observation, are of little or no value. We must await 

 the establishment of the inland Experimental Biological Station, 

 which, it is to be hoped, will one day be founded. — Ed.] 



