158 ON THE ORIGIN OF SPECIES 



of any country by no means exclusively depends on 

 insensibly changing physical conditions, but in large 

 part on the presence of other species, on which it 

 depends, or by which it is destroyed, or with which 

 it comes into competition ; and as these species are 

 already denned objects (however they may have become 

 so), not blending one into another by insensible grada- 

 tions, the range of any one species, depending as it 

 does on the range of others, will tend to be sharply 

 denned. Moreover, each species on the confines of its 

 range, where it exists in lessened numbers, will, during 

 fluctuations in the number of its enemies or of its prey, 

 or in the seasons, be extremely liable to utter exter- 

 mination ; and thus its geographical range will come 

 to be still more sharply defined. 



If I am right in believing that allied or represent- 

 ative species, when inhabiting a continuous area, are 

 generally so distributed that each has a wide range, 

 with a comparatively narrow neutral territory between 

 them, in which they become rather suddenly rarer and 

 rarer ; then, as varieties do not essentially differ from 

 species, the same rule will probably apply to both ; and 

 if we in imagination adapt a varying species to a very 

 large area, we shall have to adapt two varieties to two 

 large areas, and a third variety to a narrow intermediate 

 zone. The intermediate variety, consequently, will 

 exist in lesser numbers from inhabiting a narrow and 

 lesser area ; and practically, as far as I can make out, 

 this rule holds good with varieties in a state of nature. 

 I have met with striking instances of the rule in the 

 case of varieties intermediate between well-marked 

 varieties in the genus Balanus. And it would appear 

 from information given me by Mr. Watson, Dr. Asa 

 Gray, and Mr. Wollaston, that generally when varieties 

 intermediate between two other forms occur, they 

 are much rarer numerically than the forms which they 

 connect. Now, if we may trust these facts and infer- 

 ences, and therefore conclude that varieties linking 

 two other varieties together have generally existed in 

 lesser numbers than the forms which they connect, 



