XXvi INTRODUCTION. [CH. 



product of a large individual of this kind. If this rate 

 of increase were unchecked, our seas and rivers would in 

 a comparatively short time (reckoning geologically) be 

 filled up with the remains of shell- fish. 



Hybridism. — Although many surmises have from time 

 to time been hazarded as to the production of abnormal 

 forms of Mollusca by means of an unnatural union be- 

 tween individuals of different species, the only direct 

 experiments or observations that appear to have been 

 published on the subject have been made by French 

 naturalists. M. Gassies, in his descriptive Catalogue of 

 the Land and Freshwater Mollusca found near Agen, 

 mentioned several cases of what he called " accouple- 

 ments adulterins/^ which he had observed between in- 

 dividuals of Helioo virgata and H. Pisana, as well as 

 between those species and Bulimus decollatus. M. Gassies 

 enclosed the snails during a thunderstorm in a vessel 

 covered ^^At\\ metallic gauze ; and he believed that the 

 electricity with which the air was then charged induced 

 the unnatural union. Great care appears to have been 

 taken to prevent any error in the result, by selecting 

 individuals which had not been previously fertilized and 

 keeping them after fecundation separate from any others. 

 The product of these unions was as follows. The young 

 of H. Pisana had perfectly white shells, — their mother 

 li^ving the usual coloured bands ; and the young of H. 

 virgata had shells of a darker colour than that of their 

 mother. In the other case, the product of the Helices 

 which had been coupled with the Bulimus was various. 

 Many had shells which were almost scalariform ; the 

 shells of others were pyramidal ; but the greater part of 

 them had shells exactly like that of their mother. The 

 product of the Bulimus did not differ from their maternal 

 form. M. Gassies had also observed the product of a 



