500 LAND ANIMALS 



to be due to their adaptation to low temperatures by means of the 

 dense hairy covering of their bodies (cf. p. 394 and 509). 



Mountain tops, especially in the tropics, may swarm with flying 

 insects apparently carried by the winds, since they neither develop 

 there nor feed there as adults. Arachnids are abundant in the high- 

 lands, especially spiders, harvestmen, and mites. Hingston found an 

 attid spider at 22,000 ft. (6710 m.) on Mount Everest. 35 



The snails of the alpine zone, like the insects, consist of a majority 

 of widespread forms and a minority of alpine species. Of 90 forms 

 in the alpine zone of the Tirol, 24 are alpine-limited. Moisture- and 

 cold-tolerant forms predominate, and bush and tree snails are absent. 

 The stenothermal cold-tolerant species of the genus Vitrina are wide- 

 spread in the mountains of the Old and New Worlds, ranging up to 

 3000 m. in the Alps and to 4400 m. on Kilimanjaro. 42 Rock snails of 

 the genera Clausilia and Campylaea are abundant. Reduction in size 

 of the shell is frequent with increasing altitude of the habitat of the 

 specimens. 43 The snow-zone Vitrina annularis and V. nivalis appear to 

 be dwarf forms of V. pellucida and V. diaphana.** Snails of the same 

 species are about twice as large at 2800 m. near Chini, in Tibet, as at 

 4000 m. near Spiti. 45 



The presence of amphibians at high altitudes depends on the pres- 

 ence of sufficient moisture. In the higher parts of Tibet, where snails 

 are extremely scarce on account of the dryness of the climate, frogs 

 and salamanders are absent, 46 though both range to great altitudes 

 on the Tibetan border, in western Szechwan. In the humid Paramo zone 

 (= alpine zone) of the Andes, frogs of the genus Eleutherodactylus are 

 abundant to the snow line at 4550 m. 47, 48 The highest known occur- 

 rence of an amphibian is that of the green toad, Bufo viridis, at 5000 m. 

 in the Himalayas. In the Alps the altitudes reached by frogs and toads 

 depend on the length of their larval life, corresponding to the period 

 during which the breeding pools are free from ice, which decreases 

 with altitude. This is shown in the table 49 given in the footnote.* 



Rana temporaria has no external adaptations to life at these high 

 altitudes. It has a relatively larger heart than Rana esculenta, which 

 is confined to the warmer lowlands, which may give it an advantage 



