INFLUENCE OF EXTENT OF RANGE 125 



tions in the range of an animal undergo decided changes which tend 

 to kill off specialized forms unable to meet these particular changes. 

 Restriction of range. — The limitation of the range of a species 

 may be extreme. Small islands, high mountain ranges or peaks, moun- 

 tain valleys, and other sharply denned areas may have species confined 

 to them. Examples among Lepidoptera are to be seen in the Attacus 

 isabellae, known only from a forest near Madrid; the notodontid 

 Rhymato-phila alpirm, known only from the neighborhood of Digne in 

 southern France; the hawk moth Akbesia davidis, from Akbes in 

 Syria; and the genus Zygaena, with nearly 200 closely similar forms, 

 most of them in the Mediterranean district and many with very small 

 ranges. The flight area of Z. seriziata in Algeria is sometimes restricted 

 to an area 8 to 10 meters broad. 13 Many flightless carabid beetles 

 are restricted to particular mountain ranges or to parts of them; 

 Carabus adonis is known only from the Parnassus and Taygetus, and 

 C. olympiae only from the valleys of Aosta and Sesia in the Apen- 

 nines. 14 Among snails, Limnaea involuta is restricted to a small moun- 

 tain lake in Ireland, and Clausilia sealaris is found only in a small cal- 

 careous area in Malta; one mollusk genus Lanzia is confined to the 

 moss of a mountain peak in Bourbon. 15 Among birds a restricted dis- 

 tribution is common among hummingbirds, as in Oreotrochilus chim- 

 borazo chimborazo from Chimborazo and Eriocnemis glaucopoides 

 from Valle Grande in Bolivia. 



The combination of slight vagility with the presence of barriers to 

 distribution usually conditions the stenotopy of a species. It seems 

 likely, however, that stenotopic forms will be found to have in common 

 other characteristics which influence the limitation of their range. 

 Various reasons for limited distribution may be considered: slight 

 power of expansion, high degree of specialization in adaptation to spe- 

 cial conditions, youth of the species, or age and decline. Low vagility 

 alone does not require consideration on account of the amount of 

 time which has been available. The parts of Germany which were 

 covered with ice during the glacial period have been completely re- 

 populated with such slow-moving forms as snails and earthworms. The 

 power of expansion possessed by a species must in the main depend 

 upon its fertility, but no example of restricted range certainly assign- 

 able to a low degree of fertility can be cited. New forms, in general, 

 will arise in quite definitely circumscribed areas, as is to be seen in 

 the melanistic forms of the Lepidoptera Boarmia consonaria and 

 B. consortaria in the neighborhood of Maidstone in England. 16 



Three suggestions have been made regarding the mechanism of 

 origin or the reason for success of the melanic variety, doubledayaria, 



