THE ORIGIN OF SPECIES AND GENERA 271 



the variation of species in existing Nature has 

 already shown us the important part played by 

 geographical limitation in the creation of varieties 

 or local races, particularly numerous among the 

 Molluscs of terrestrial or flu vio-lacus trine habitat. 

 We have seen, with Neumayr, the remarkable 

 examples of limitation furnished by the Achatinellae 

 of the Sandwich Islands, the Iberi of Sicily, and 

 the Melanopses of the Mediterranean basin. In these 

 three groups the divergence of the extreme forms is 

 so marked that no naturalist hesitates, notwith- 

 standing the existence of intermediate forms, to 

 recognize in them perfectly distinct species. A few 

 malacologists have even proposed to recognize in 

 the Melanopses the formation of three genera, 

 closely related, it is true. Isolation in islands con- 

 stitutes for terrestrial animals, whether Vertebrates 

 or Invertebrates without means of aerian locomotion, 

 one of the most favourable conditions for the di- 

 vergence of local forms. One of the most remarkable 

 cases of this is assuredly that of the giant land Turtles, 

 which constitute two groups of species, nearly every 

 one of which is peculiar to one of the islands of 

 the Archipelagos of the Mascareignes and of the 

 Galapagos. Doubtless each of these groups repre- 

 sents the differentiation of the same original type 

 gradually modified by insular isolation. 



The modifying influence of isolation is also 

 very easy to observe among our fresh- water animals, 

 whether of rivers or of lakes. The species of Unios, 

 Anodonts, and Limnaeas are often strictly confined 

 to the same hydrographical basin. All naturalists 

 are acquainted with the great morphological and 



