OF TRANSITIONAL VARIETIEB. 161 



and sometimes it is quite remarkable how abruptly, as 

 Alph. de OandoUe has 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 climate 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 ea.ch organic being is either directly 

 or indirectly related in the most important manner to 

 other organic beings — we see that the range of the inhab- 

 itants of any country by no means exclusively depends on 

 insensibly changing physical conditions, but in a large 

 part on the presence of other species, on which it lives, or 

 by which it is destroyed, or with which it comes into com- 

 petition; and as these species are already defined objects, 

 not blending one into another by insensible gradations, the 

 range of auy 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 les- 

 sened numbers, will, during fluctuations in the number of 

 its enemies or of its prey, or in the nature of the seasons, 

 be extremely liable to utter extermination; and thus its 

 geographical range will come to be still more sharply 

 defined. 



As allied or representative species, when inhabiting a 

 continuous 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 es- 

 sentially 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 intermediate variety, consequently, will exist in lesser 

 numbers from inhabiting a narrow and lesser area; and 

 practically, as far as I can make out, tliis rule holds good 

 with varieties in a state of nature. I have met with strik- 

 ing instances of the rule in the case of varieties intermediate 



