TRANSITIONAL VARIETIES 181 



observed, a common alpine species disappears. The same 

 fact has been noticed by E. Forbes in sounding the depths of 

 the sea with the dredge. To those who look at climate and 

 the physical conditions of life as the all-important elements 

 of distribution, these facts ought to cause surprise, as cli- 

 mate and height or depth graduate away insensibly. But 

 when we bear in mind that almost every species, even in its 

 metropolis, would increase immensely in numbers, were it not 

 for other competing species; that nearly all either prey on or 

 serve as prey for others ; in short, that each organic being is 

 either directly or indirectly related in the most important 

 manner to other organic beings, — we see that the range of the 

 inhabitants of any country by no means exclusively depends 

 on insensibly changing physical conditions, but in a large 

 part on the presence of other specie?, c:-' which it lives, or by 

 which it is destroyed, or with which it comes into competi- 

 tion; and as these species are already defined objects, not 

 blending one into another by insensible gradations, the range 

 of any one species, depending as it does on the range of 

 others, will tend to be sharply defined. Moreover, each 

 species on the confines of its range, where it exists in less- 

 ened numbers, will, during fluctuations in the number of its 

 enemies or of its prey, or in the nature of the seasons, be ex- 

 tremely liable to utter extermination; and thus its geographi- 

 cal range will come to be still more sharply defined. 



As allied or representative species, when inhabiting a con- 

 tinuous area, are generally distributed in such a manner 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 take a varying species inhabiting 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 intermedi- 

 ate 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 vari- 

 eties in the genus Balanus. And it would appear from infor- 



