10 J. T. Gulick — Inconsistencies of Utilitarianism. 



the isolation is produced 'may be in itself segregative, in that 

 it brings together those endowed in some special way, causing 

 them to breed together, and preventing them from breeding 

 with others. This is especially the case with Sexual, Social, 

 and Prepotential, Segregation, and in some degree with Indus- 

 trial Segregation. Isolation thus produced is in its very nature 

 segregative, and would result in divergence if diversity of 

 natural selection did not arise in the different sections of the 

 species. Segregation with divergence may also be produced 

 by natural selection or some other principle of transformation 

 co-operating with some form of isolation that of itself is not 

 perceptibly segregative. As segregation of other than average 

 forms always produces divergence, and without it there is no 

 divergence, I claim that it is the fundamental principle of 

 divergent or polytypic evolution. Natural selection, which is 

 the exclusive propagation of those better adapted to the envi- 

 ronment, when it results in the preservation of other than 

 average forms, produces confluent or monotypic evolution ; 

 but it is never the cause of divergence, except when co-operat- 

 ing with some principle of isolation in such a way that the 

 two principles produce segregation. Failure to recognize 

 these distinctions, prevents Mr. Wallace from understanding 

 my theory, and leads him to represent me as claiming for isola- 

 tion all that I claim for segregation. 



Incompatibilities arise during Positive Segregation. 



On pages 173-186, Mr. Wallace maintains that "Natural 

 selection is, in some probable cases at all events, able to accu- 

 mulate variations in infertility between incipient species" 

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

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

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

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

 tive modification of varieties, races or species, selection, whether 

 artificial or adaptational, must preserve certain forms of an 

 intergenerating stock, to the exclusion of other forms of the 

 same stock. Progressive change in the 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 modi- 

 fies 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 



