GEOGRAPHIC ISOLATION 93 



insignificant change of locality may yield a slight but constant change ; 

 the geographic races are accordingly innumerable. Other American 

 Lepidoptera, such as Heliconius, Melinaea, and Mechanitis, have a 

 similar behavior and distribution. The erycinid genus Calydna differs 

 from its relatives in the possession of strong powers of flight, and in 

 this genus the individuals of a species, even from widely separated 

 localities, exhibit an appreciable variation. 77 Such geographic races 

 especially of mammals and birds frequently exhibit no morphological 

 differences and are distinguished primarily by means of coloration. 

 Constancy of a character in a series of specimens, rather than the 

 degree of difference, forms the basis for the recognition of subspecies 

 in the practice of many American taxonomists. 



Isolation and primitive forms. — Another effect of geographic iso- 

 lation is the preservation of groups from destruction by removing 

 them from competition with their more advanced relatives. Remnants 

 of certain species, genera, families, and even orders, which formerly 

 had a wide distribution, have been able to maintain themselves in 

 isolated areas when the isolating barriers prevented the entry of the 

 more advanced competitors. 



Thus Lepus timidus, elsewhere a mountain and arctic form, occurs 

 at all levels in Ireland, where it is free from the competition of the 

 European hare, Lepus europaeus. 18 In Germany, where the two lizards 

 Lacerta agilis and Lacerta vivipara occur together, the former devours 

 the young of the latter, and thus prevents its spread. L. vivipara is 

 able to maintain itself only in areas where its enemy cannot live, 

 especially in places without suitable sites for egg-laying or where the 

 temperatures are too low to hatch them. Mountains above 1200-1500 

 m., the arctic territory, and bogs and swamps, even south of the Alps 

 (for example, in Lombardy), furnish these conditions, and these are 

 inhabited by L. vivipara. Planaria alpina is in competition with 

 Planaria gonocephala and Polycelis cornuta wherever the water tem- 

 perature has a range of variation of more than 6°C. At intermediate 

 heights P. alpina is found only in the uppermost parts of spring brooks; 

 at altitudes where its competitors are excluded by the uniform coldness 

 of the water, it is widely distributed. 



Disappearing races of mankind have been crowded together in 

 much the same way in unfavorable and inaccessible areas. A large 

 number of languages are found in the Caucasus, not only unrelated 

 among themselves, but also unrelated to other known languages. There 

 seems to be an accumulation of remnants of linguistic stocks. Forty 

 of the 59 language groups of North America are similarly represented 

 between the Pacific and the Rocky Mountains. In South America an 



