152 ABSENCE OR RARITY 



mensely 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 species, on which it lives, or by which it is destroyed, 

 or with which it comes into competition ; and as these 

 species are already denned objects, not blending one into 

 another by insensible gradations, the range of any one spe- 

 cies, depending as it does on the range of others, will tend 

 to be sharply denned. Moreover, each species on the con- 

 fines 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 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 

 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 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 inter- 

 mediate 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 inferences, 

 and conclude that varieties linking two other varieties to- 

 gether generally have existed in lesser numbers than the 

 forms which they connect, then we can understand why in- 

 termediate varieties should not endure for very long periods : 

 why, as a general rule, they should be exterminated and 

 disappear, sooner than the forms which they originally linkecl 

 together. 



