234 APPENDIX II—INTENSIVE SEGREGATION. 
have not been seriously changed since the close of the last glacial 
period. Again, one generation of the seventeen-year race of Cicada 
covers many generations of the Basilarchia, bringing thirty or forty 
fluctuations of climate, food, etc.,to the latter, while the former is, for 
the most part, protected from serious fluctuations. 
It is of course equally impossible to prove by allinclusive observa- 
tions either that transformation is never completely parallel in sections 
of a species that are prevented from crossing or that independent gene- 
ration long continued is sure to result in independent transformation, 
and, therefore, in divergence; but it is of no small interest that we 
find in the thirteen-year and seventeen-year races of this species the 
strongest proof that there are sometimes divergences which our senses 
do not perceive. If our senses were a sufficient test, it might be main- 
tained that between these races a high degree of loeal and cyclical 
isolation has existed for many generations, without any other form of 
transformation having arisen to increase the divergence; but if our 
informants are correct when they tell us that these races do not cross 
when appearing in the same district and at the same time, we need not 
hesitate to affirm that there must be some distinguishing character- 
istics by which those of one race are able to find each other, as well 
as segregative instincts which lead them to choose each other’s 
society; and, even if our informants are mistaken in supposing that 
cross-unions do not occur, there must be some form of incompatibility 
between the two races, resting on divergent endowments; for other- 
wise we should find hybrid descendants with periods of more than 
thirteen and less than seventeen years’ duration. 
IV. CONCLUDING REMARKS. 
1. Outline of the Argument in Support of the Theory of Divergent Evolution 
through Cumulative Segregation. 
(1) The invariable experience of mankind in producing domestic 
races shows that segregation is a controlling factor. The segregation 
that produces domestic breeds and races is found to be of:two kinds: 
first, that which is produced by men who designedly*preserve the 
different styles of variation presented by one species, while at the 
same time they prevent them from crossing; and, second, that which 
commences in the indiscriminate division of the species into sections 
that are prevented from freely crossing through their being under the 
care of separate tribes of men, and which is changed into decided 
segregation through the diversity of selection, or of some other trans- 
forming principle, to which the different sections are sure to be 
