THE LOCAL EXTINCTION OF MOLLUSCS. 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 virgata. 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
virgata, 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 difficult 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.]