CHAPTER VIII 

 THE INFLUENCE OF EXTENT OF RANGE 



The area inhabited by a species (or any phyletic unit) is termed 

 its range. The range of an animal is delimited by the lines connecting 

 the outermost localities at which it is found. A species need not occupy 

 the whole extent of its range; for the most part it will be found only 

 in special habitats which fulfill specific conditions. The extent of a 

 range depends quite as much upon the presence of similar habitats and 

 upon the arrangement of barriers as upon the history and character of 

 the species. The ranges of animals of different groups are accordingly 

 highly unlike. The extent or limitation of the range of a species may 

 have an important effect upon the selection and constitution of its 

 individuals. Such regularities as are discoverable in these factors are 

 the subject of this chapter. 



A subspecies, species, genus, family, etc., which inhabits a small 

 range may be referred to as stenotopic, one with a wide range as 

 eurytopic. The extent of a range depends upon a number of factors, 

 including especially the geologic age and variability, the vagility, and 

 the ecological valence of the group concerned. The existing barriers, 

 of course, prescribe a maximum to the range of any group. A species of 

 frog on an oceanic island or a carabid beetle in a cave are usually 

 restricted in range to this island or cave. Their range, however, may 

 be restricted to a special part of the island or cave by other factors. 



The range of an individual species is not fixed, but fluctuates with 

 the passage of time, increases, diminishes, or may be shifted as a 

 whole. The boarfish, Capros aper, of the Atlantic and Mediterranean 

 was formerly rare on the coast of England, but about 1888 became 

 so abundant that its numbers were troublesome to the fishermen. The 

 owl butterfly, Plusia moneta, was unknown in England before 1890, 

 but since then it has become one of the common English species. 

 Merops apiaster, a bird belonging to the family of bee eaters, has 

 frequently entered south Germany, and has failed to become estab- 

 lished only because its conspicuous appearance invites persecution on 

 the part of man. The cardinal (Cardinalis cardinalis) is at present 

 actively extending its range in the Mississippi valley. The great exten- 



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