Inconsistencies of UtiUtariamsm. 135 



(p. 174) ; but his reasoning does not seem to me conclusive. 

 Even if we grant that the increase of tliis character occurs by 

 the steps wliich he describes, it is not a process of accumula- 

 tion by natural selection. In order to be a means of cum.ula- 

 tive modification of varieties, races, orspecies, selection, whether 

 artificial or adaptational, must preserve certain forms of an 

 intergenerating stock, to tiie exclusion of other forms of the 

 same stock. Progressive change in tlie size of the occupants 

 of a poultry-yard may be secured by raising only bantams the 

 first, only common fowls the second, and only Shanghai fowls 

 the third year : but this is not the form of selection that has 

 produced the different races of fowls. So in nature rats may 

 drive out and supplant mice; but this kind of selection 

 modifies neither rats nor mice. On the other hand, if certain 

 variations of mice prevail over others through their superior 

 success in escaping their pursuers, then modification begins. 

 Now, turning to p. 175, we find that in the illustrative case 

 introduced by Rlr. Wallace the commencement of infertility 

 between the incipient species is in relations to each other of 

 two portions of a species that are locally segregated from the 

 rest of the species, and partially segregated from eacli other 

 by different modes of life. These two local varieties, by the 

 terms of his supposition, being better adapted to the environ- 

 ment than the freely interbreeding forms in other parts of the 

 general area, increase till they supplant these original forms. 

 Then, in some limited portion of the general area, there arise 

 two still more divergent forms, with greater mutual infertility 

 and with increased adaptation to the environment, enablino- 

 them to prevail throughout the whole area. The process here 

 described, if it takes place, is not modification by natural 

 selection. The natural selection of which he speaks does not 

 arise till, with each advancing step, a new and complicated 

 adjustment (which introduces the two new forms, each with 

 unabated fertility with its own kind, but with diminished 

 fertility with the other kind) has been attained by some other 

 process. That other process is the one described in the passage 

 1 have already quoted from pp. 184-185, where, according to 

 my apprehension, the cause of divergence is more correctly 

 stated than it is in the passage now under consideration. In 

 the latter part of my paper on " Divergent Evolution through 

 Cumulative Segregation " I have shown that the different 

 kinds of incompatibility, preventing complete fertility between 

 incipient species (and there called forms of Negative Segre- 

 gation), cannot arise except as accompaniments of Positive 

 Segregation in some form ; but that, having once arisen in 

 connexion with partial Positive Segregation, they increase 



