THE LOCAI, EXTINCTION OF MOLLUSCS.' 87 



On looking over the list of the Land-Mollusca of the British 

 Isles, we observe how large is the proportion of small, and even 

 minute, forms. Yet other things being equal, in power of distri- 

 bution by travel the larger Molluscs have a great advantage. I 

 once kept a specimen ot Helix pomaiia, which was a habitual 

 traveller, and once crept nearly a hundred yards in one night. 

 Hence, the large species would have a better chance of obtaining 

 new settlements than small ones. It is probable that the larger 

 forms, in some cases, sufTer, or liave suffered, destruction chiefly 

 on account of their conspicuousness. It is a common expe- 

 rience of the snail-hunter that at the outset he cannot find the 

 minute forms, nor, if he had to depend upon his eyes and hands 

 alone, could he make much progress, except accidentally. Yet 

 in time he gets to learii that, in point of numbers, many of these 

 minute species may far outnumber the larger forms with which 

 ever\'one is familiar. The inference here is plain — that, given 

 any snail-eating bird or other animal, the larger forms are those 

 that will first succumb, whilst the smaller species will escape 

 notice. Part of the disparity in numbers between large and 

 small species may be due directly to their size. There are a 

 few species that are quoted chiefly from sea-board districts that 

 may be noticed. That they are not dependent upon a littoral 

 vegetation or conditions is clear from the circumstance that they 

 are sometimes found inland. Such are Succinea oblonga. Helix 

 revelata, H. pisana, H. virgata," and others. The reason seems to 

 be that they meet with enemies as the country is penetrated, and 

 rarely succeed in permanently spreading themselves inland. 



It is, however, possible that the enemies of snails in 

 England now do not very greatly affect their distribution. 

 Helix nemoyalis is everywhere common, and yet it is everywhere 

 sought for by blackbirds and thrushes. In the immediate past, 

 as instanced by the shell-marls, it was equally common, and it 

 has therefore held its own in spite of its enemies, other causes 

 probably operating to favour its vigorous life and wide distribu- 

 tion as a species. One effective cause of extinction of Molluscs 

 is change in the character of the habitat. The variations in my 

 neighbourhood (Feistead) are due to two causes. One is the 

 wasting away of a calcareous soil, and the other is the increasing 

 dryness of the soil due to drainage. We will take the cases due 

 to calcareous waste first. One is so patent as to have attracted 



2 [But Helix virgata is one of the species which Mr. French has observed immigiating 

 into Feistead. See his remarks below on the introduction of species. — Ed.] 



