394 THE INFLnEN'CB OF LIVING SURROUNDINGS. 



land-snails' as have, like the great vino snail {Helix pomatia), 

 a viride mouth to their shells are extraordinarily variable. 

 Genera which had been constituted merely on our knowledge of 

 these variable shells, such as Helix, Bulimus, Vitrina, Nanina, 

 &c., have proved quite untenable, and we now know that species 

 which, by a comparison of the animals, must be placed actually 

 in different families, often have shells so exactly alike that 

 cotichologists and palaeontologists — the latter having, of course, 

 nothing but the shells to judge from — ^have placed them in the 

 same genus. 



These recent investigations have, moreover, proved that the 

 great majority of the genera of land mollusca have very narrow 

 limits of distribution, so that with regard, for instance, to the 

 numerous shells resembling Vitrina which have hitherto been 

 described, their local origin supplies a far surer index as to 

 their affinities of relationship to this or that genus than the 

 characters of the shell itself. Setting aside a small number of 

 cosmopolitan and for the most part minute forms, most of them, 

 and particularly the larger kinds, are highly characteristic of 

 the different countries where they are indigenous. Thus the 

 three genera, Cocldostyla, Helicarion, and Rhysota, are quite cha- 

 racteristic of the Philippines ; for only a few of the species extend 

 into the neighbouring islands of the Moluccas, while they occur 

 in a very great variety of differently characterised species in the 

 Philippines themselves. Pfeiffer, who as a conchologist was 

 beyond a doubt the highest authority, included the species of 

 Cochlostyla in three different genera, and those of Helicarion in 

 two; but anatomical investigation has proved tome that the 

 species of these three genera, in spite of the gi'eat diversity of 

 their shells, are quite as much alike as the different races of the 

 Germanic or Romance nationality. 



The species of these three characteristically Philippine gsnera 

 are mimicked in a very remarkable manner, both in form and 

 colour, by species of other genera which are not characteristic of 

 the Philippines only, but of the neighbouring groups of islands 

 as well ; one of these cases is, beyond a doubt, one of the most 

 striking instances of true mimicry. 



The animals of the species of Helicarion (see fig. 105, c)— of 



