VARIATION IN SPACE AT PRESENT EPOCH 131 



membered by modern authors. Let us take for 

 an example a species of moderate polymorphism, 

 the Bulimus detritus, a pretty shell very common in 

 the south-east of France. The variations of 

 this species may rest 



1. On the size of the shell : modes major, medius, 

 minor. 



2. On its more or less inflated form : modes in- 

 flatus, normalis, elongatus. 



3. On its white, striped, or semi-transparent 

 colour : modes albidus, radiatus, Cornells. 



4. On the coiling of the spiral : modes regularis, 

 irregular is. 



5. On the smooth or rough aspect of the epidermis: 

 modes Icevigatus, excoriatus. 



These thirteen modes, to which a few others 

 might be added, are not met with in all the stations 

 of this species ; each colony may be characterized, 

 on the contrary, by the absence, the presence, or 

 the frequency of one or other particular mode. 

 In one, for instance, all the individuals may be 

 long, mode elongatus ; in another all very small, 

 mode minor. Often there are combinations, two 

 to two, or three to three, of these different modes: 

 In the Bulimus detritus most of these modes are met 

 with here and there in different places, without any 

 very precise geographical distribution. However, 

 the mode corneus is almost special to Auvergne 

 (Bulimus corneus), and the association of the modes 

 minor, elongatus, and irregularis characterizes a 

 form quartered on the east slope of Mont Lepine 

 in Savoy, which has received from Bourguigniat 



the name of Bulimus sabaudinus. These two 



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