280 BARRIERS TO DISPERSAL chap. 



little effect in checking dispersal. There is no appreciable 

 difference between the land Mollusca north and south of the 

 Ganges, or north and south of the Amazon. Living snails, or 

 their ova, are no doubt transported from one bank to another on 

 floating debris of various kinds. The barrier offered by the sea 

 is obvious, and at first sight appears insurmountable ; but the 

 facts with regard to oceanic groups of islands like the Azores 

 and Canaries (see p. 297) show that even a stretch of salt w^ater 

 many hundred miles in breadth may be ineffectual in preventing 

 the dispersal of Mollusca. 



Mountain ranges, provided they are too high to be scaled, 

 and too long to be turned in flank, offer a far more effective 

 barrier than the sea. Every thousand feet upward means a fall 

 of so many degrees in the mean temperature, and a change, 

 more or less marked, in the character of the vegetation. There 

 is generally, too, a considerable difference in the nature of the 

 climate on the two sides of a great mountain range, one side 

 being often arid and cold, the other rainy and warm. The 

 combined effect of these influences is, as a rule, decisive 

 against the dispersal of Mollusca. Thus the Helices of Cali- 

 fornia are almost entirely peculiar ; one or two intruders from 

 states farther east have succeeded in threading their way 

 through the deep valleys into the Pacific provinces, but not a 

 single genuine Californian species has been able to scale the 

 heights of the Cascade Mountains. The land Mollusca of India 

 are numbered by hundreds; not one penetrates north of the 

 Himalayas. According to Mr. Nevill,^ the change from the 

 Indo-Malayan to the so-called European molluscan fauna at 

 the northern watershed of the Kashmir valley is most abrupt 

 and distinct; in two days' march northward, every species is 

 different. Ranges of inferior altitude, such as the P3Tenees, 

 the Carpathians, or the Alleghanies, may be turned in flank as 

 well as scaled, and we find no such marked contrast between 

 the Mollusca on their opposite sides. 



The most effective barrier of all, however, is a desert. Its 

 scorching heat, combined with the absence of water and of 

 vegetable life, check dispersal as nothing else can. The distri- 

 bution of the Mollusca of the Palaearctic Region is an excellent 

 instance of this. Their southern limit is the great desert which 

 1 Scient. Besults Sec. Yarkand Exped. " Mollusca," pp. 1-16. 



