396 LAND ANIMALS 



strengthened, as for example, in man, by pigmentation of the sheaths 

 of the central nervous systems; and in fact the pigment in the pia 

 mater of brunette Europeans and brown Egyptians is more abundant 

 than in that of the black races, as for example, the Sudan negroes. 62 

 Sun-dwelling geckos such as Lygodactylus picturatus, have a black 

 pigmented peritoneum, while in those with the nocturnal habit, like 

 Hemidactylus turcicus, it is unpigmented. 63 Life in direct sunlight is 

 made possible only by certain protective adaptations ; amphibians and 

 reptiles, which often sun themselves (Hyla, Lacerta) , have some pro- 

 tection against the penetration of radiation into the body. 64 



Pressure and air currents. — Atmospheric pressure and its fluctua- 

 tions are of significance only for the animals of high mountains and 

 will be discussed in Chapter XXIV. 



Strong air currents exert a selective effect upon animals in a special 

 way, and flying animals— insects, birds, and bats— are particularly 

 affected. Strong winds carry off flying animals. Every strong conti- 

 nental wind delivers millions of coastal butterflies to the sea, and then 

 when the sea wind suddenly arises their bodies are washed up on the 

 beach in large numbers. On the Frisian Islands many insects that occur 

 on the neighboring coasts are absent, probably because of the selection 

 that the storm winds effect ; the butterflies Pieris, Hipparchia, Vanessa, 

 and the flies Eristalis, Anthrax, Empis, for example, 65 do not occur in 

 these islands. 



In regions where strong winds prevail, certain peculiarities may be 

 noticed among the flying animals: either the ability to fly must be 

 stronger, or, more frequently, it is entirely lost. On the treeless, stormy 

 Faeroes the starling Sturmis vulgaris, a permanent resident, has longer 

 and stronger wings and a longer tail than its relatives elsewhere. 66 

 Butterflies copulate in the air, and this habit causes them to be more 

 dependent on the wind condition than other insects; hence only the 

 sturdy and strong-flying butterflies are found on such islands as the 

 Marshall Islands. 67 Nineteen of the most beautiful butterfly genera at 

 Ega on the Amazon River, with about a hundred species, are not 

 present farther downstream in reach of the sea winds. 68 Most striking, 

 however, is the frequent occurrence in stormy regions of insects with 

 more or less degenerate wings. On the subantarctic islands, as on the 

 Kerguelen and Crozet islands, the South-Georgian and Auckland is- 

 lands, where calm days are the exception, the majority of insects have 

 reduced wings or no wings at all (see Chapter XXVI) . 



On the other hand, many birds in their flight utilize air currents, 

 both vertical and horizontal. The effect of rising air currents may be 

 observed on steep coasts during sea winds, which strike the slope and 



