76 ECOLOGICAL FOUNDATIONS OF ZOOGEOGRAPHY 



Isolation is also to be seen in related forms which inhabit the 

 same region, but occur in different environments and have different 

 habits; this constitutes ecological isolation. This is illustrated by two 

 closely related crayfishes that occur together in southwestern Penn- 

 sylvania. One, Cambarus monongalensis, is confined to springs with 

 clear water; the other, C. diogenes, lives in marshes and other stag- 

 nant waters. 6 



Closely related forms, which now inhabit the same area, may of 

 course have originated in separate areas and have come together by 

 migration. They will then remain separate if a sufficient degree of 

 bodily or instinctive differentiation to prevent interbreeding has arisen 

 during their separation. The animals of the Chicago area, for example, 

 have entered from three different directions since the glacial period; 

 southwestern (Sonoran), southeastern, and northern elements are dis- 

 tinguishable. The fauna of central Europe has a similarly diverse 

 origin, and Taylor writes of its grasshoppers: "This circumstance 

 explains some cases in which two or three very closely allied forms 

 inhabit the same area and occur in the same habitat. They have 

 originated in the three distinct areas from a preglacial common ances- 

 tor, and have reached their present common area of distribution only 

 after postglacial migration." 7 The European wheatear, a characteristi- 

 cally Eurasian thrush, ranges through most of Eurasia, and has entered 

 North America from two different directions. Saxicola oenanthe leu- 

 corhoea reaches Labrador via Greenland, while S. oe. oenanthe has 

 reached Alaska from the Chuckchen Peninsula. 8 It is a matter of only 

 a relatively short time until the two forms will meet in Arctic 

 America. 



The same barrier will be of very different effectiveness for different 

 species. The power of dispersal of an animal species may be summed 

 up as its "vagility." The less the vagility of a species, the less it is 

 able to overcome barriers, the more numerous are the areas which 

 afford the condition of geographic isolation for it, and hence the more 

 numerous the opportunities for independent variation. The degree of 

 vagility of different groups of animals is in inverse proportion to the 

 number of geographic races presented by a given area. 9 The crested 

 lark, Galerida cristata, a strictly resident form which rarely ranges 

 far from its home, forms an unusually large number of local races. 

 The gray sea eagle scarcely varies with a nearly world-wide range. 

 Widely distributed cirripedes tend to break up into local races, unless 

 they lead a planktonic existence by attaching to floating wood and 

 pumice, like most species of Lepas and Conchoderma. 10 In the same 

 area a species of high vagility may be represented by a single form, 



