134 
Rev. J. T. Gulick on uie 
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 principles 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 principle 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 w^ay 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. 
Incompatibilities arise during Positive Segregation . 
On pages 173-186 Mr. Wallace maintains that u Natural 
selection is, in some probable cases at all events, able to accu- 
mulate variations in infertility between incipient species ” 
