INFLUENCE OF EXTENT OF RANGE 129 



Many widely distributed species exhibit the interesting peculiarity 

 that they tend to develop numerous restricted varieties at the borders 

 of their range while remaining constant over wide areas in the center. 

 This is true of the snail Helix aspersa, which shows unusual variation 

 at its southern limit, in Algeria. The same is true of Helix (Arianta) 

 arbustorum, which varies strongly at both the horizontal and vertical 

 limits of its range (for example, in the Pyrenees) and of the "Formen- 

 kreis" Levantina in Palestine. 25 Albinism is especially common at the 

 limits of the specific range, as in H. arbustorum in the Alps, and 

 Helix {Cepaea) nemoralis, which is rarely albinistic in Germany, fre- 

 quently has albinos in the southern part of its range, in Italy. 26 



The European newts sometimes have constant dwarf races in the 

 south; males of Triturus palmatus from Porto, Portugal, measure 55 

 mm., compared with 70-80 mm. north of the Pyrenees. 27 The common 

 pond frog of Eurasia (Rana esculenta) is represented in the central 

 part of its range by the subspecies ridibunda, while the forms at the 

 west and the southwest (the typical form and the subspecies lessonae) 

 differ so much from each other and from the Chinese representative, 

 Rana esculenta nigromaculata, that an observer with only the Japa- 

 nese and Spanish forms before him would not hesitate to consider 

 them as distinct species. 28 



Another instance of this relation between distribution and vari- 

 ation is presented by the European birds called tits. The least spe- 

 cialized form, Parus pleskii, occurs in central Russia, a form some- 

 what intermediate between the subspecies in the east, south and west; 

 toward the northeast and north it is replaced by Parus cyanus, in the 

 southwest and west by P. caeruleus and its subspecies, and in the east 

 it reaches the range of P. flavipectus. 29 The Eurasian deer have formed 

 dwarf races at the western limits of their range in the Faeroe Islands, 

 Scotland, Alsace, Corsica, Spain, and Algeria, as well as at the eastern 

 limit in Japan. It is probable that the conditions of life at the limits 

 of the range of a given species are less favorable, and that these are 

 responsible for the change and dwarfing of the species, especially in 

 the somewhat isolated outposts which do not cross as freely with the 

 central mass of the species. In a narrow range the habitat conditions 

 will in general afford less variety and the separation of the individual 

 stations at which the species occur will be less. 



There is a minimum value for the extent of the range, which 

 depends upon the nature of the species and consequently varies with 

 the species concerned. Each individual animal requires a minimum 

 space in which it can satisfy its needs for food and movement. It is 

 evident that an area that would seem large to a mouse would be small 



