134 Rev. J. T. Gulick on u,^ 



complete as preventives of intercrossing. A very stable and 

 homogeneous species may be divided by geological subsidence 

 into two large sections, each represented by a vast number of 

 individuals. In such a case the difference in the average 

 character, and consequently the degree of segregation, of the 

 two sections will be infinitesimally small, and the influence of 

 the isolation thus produced will chiefly consist in its preserving 

 in the different sections any diversities that may arise in the 

 effects of natural selection or of other ])rinciples of transfor- 

 mation. The isolation between the land-animals of Ireland 

 and Britain, which Mr. Wallace cites as adverse to my theory, 

 is of this kind. Again, there may be transportation and iso- 

 lation of very small fragments of a very variable species. In 

 such a case separation may involve a degree of segregation 

 that from the first produces perceptible divergence. Again, 

 the process by which 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 pre- 

 venting them from breeding with others. This is especially 

 the case with Sexual, Social, and Prepotential Segregation, 

 and in some degree with Industrial 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 ])rinciple of transformation cooperating 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 propa- 

 gation of those better adapted to the environment, when it 

 results in the preservation of other than average forms, pro- 

 duces confluent or monotypic evolution ; but it is never the 

 cause of divergence, except when cooperating 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 isolation all that I 

 claim for segregation. 



Incompatihtlities arise during Positive Segregation. 



On pages 17H-186 Mr. Wallace maintains that "Natural 

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

 mulate variations in infertility between inci])icnt species " 



