170 ABSENCE OF RARITY Chap. VI. 



for otlicrs ; in short, that cadi organic beinf^ is cither directly 

 or indirectly related in the most important manner to other 

 organic beings, \vc must see that the range of the inhabitants 

 of any country by no means exclusively depends on insensibly 

 changing pliysical conditions, but in large part on the presence 

 of other species, on which it lives, or by ■which it is destroyed, 

 or with which it comes into comjietition ; and as these species 

 arc already defmed objects, not blending one into another by 

 insensil)le gradations, the range of any one species, depending 

 as it does on the range of others, will tend to be sharply de- 

 fined. Moreover, each species on the confines of its range, 

 wherc 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 extermination ; and thus its geo- 

 graphical range Avill come to be still more sharply defined. 



If I am right in believing that allied or representative spe- 

 cies, when inhabiting a continuous area, are generally so dis- 

 tributed that each has a wide range, with a comparatively 

 narrow neutral territory between tliem, in which they become 

 rather suddenly rarer and rarer ; then, as varieties do not essen- 

 tially 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 inter- 

 mediate variety, consequently, will exist in lesser numbers 

 from inhabiting a narrow and lesser area ; and practical!}', as 

 fiir as I can make out, this rule holds good with varieties in a 

 state of Tjature. 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 ]\Ir. "Watson, Dr. Asa Gray, and Mr. 

 Wollaston, that generally, when varieties intermediate between 

 two other forms occur, they arc much rarer numerically than 

 the forms which they connect. Now, if we may trust these 

 facts and inferences, and therefore conclude that varieties link- 

 ing two other varieties together hare generally existed in 

 lesser numbers than the forms which they connect, then, I think, 

 we can understand wh}' intermediate varieties should not en- 

 dure for very long periods ; Avhy, as a general rule, they should 

 be exterminated and disaiii)ear, sooner than the forms v>hich 

 they originally linked together. 



"For any form existing in lesser numbers would, as already 

 remarked, ran a greater chance of being exterminated tlian 



