276 DIVERGENT EVOLUTION THROUGH SEGREGATION. 
since the publication of my paper on * Diversity of evolution under 
one set of external conditions,” was for the most part written out in 
its present form. Since then, and with reference to the discussion on 
physiological selection, I have worked out the algebraic formulas given 
in the jast chapter, and have introduced explanations of the same; but 
at the same time I have removed several! chapters in which the principle 
of selection was discussed at length, and have endeavored to bring the 
whole within a compass that would allow of its being published by some 
scientific society. In order to attain this end, I reserve for another 
occasion a discussion of the principles of intensive segregation, under 
which name I class the different ways in which other principles com- 
bine with segregation in producing divergent evolution. 
It was my intention to bring together examples of the different forms 
of Segregation discussed, that they might be published with the theo- 
retical part; but the large number of pages found necessary for even 
the briefest presentation of the principles involved, and the fact that 
Mr. Romanes’s paper has appeared relating to some of the same prob- 
lems, leads me to present the results of my studies without further 
delay. The facts on which large portions of my theory rest are of the 
most familiar kind, and no additional light would be gained though 
their numbers were multiplied a hundredfold. Indeed, one of the 
marked features of my theory is that in its chief outlines it rests on 
facts that are universally acknowledged. The aim of the theory is to 
show the connection of these facts with divergent evolution. 
Though many divergencies appear in our method of treating the sub- 
ject, the fundamental theory underlying my segregate fecundity and 
Mr. Romanes’s Physiological Selection seems to be very similar, if not 
thesame. The most important differences I have noticed are, (L) that he 
seems to regard mutual sterility as sufficient to account for the separate 
propagation of species and varieties thus characterized, without calling 
in the aid of any other form of segregation, while I regard it as a neg- 
ative form of segregation that would result in the general destruction 
of all life if not associated with what I call positive forms of segrega- 
tion; and (2) that he maintains that “ Physiological selection is almost 
exclusively a theory of the origin of species, seeing that it can but 
very rarely have had anything to do with the formation of genera, and 
can never have had anything at all to do with the formation of fami- 
lies, orders, or classes. Hence the evidence which we have of the evo- 
lutionary influence of physiological selection, unlike. that which we 
-have of the evolutionary influence of natural selection, is confined 
within the limits of specific distinctions,”* while I maintain that segre- 
gation of some form is a necessary condition for all divergent evolu- 
tion, and that in fact segregate fecundity in many cases prevents the 
inter-crossing of divergent forms that, though descended from a com- 
mon stock, now belong to different families and orders. 
* Linn, Soc, Journ., Zodlogy, vol. X1x, p. 396, 
