DIVERGENT EVOLUTION THROUGH SEGREGATION. PALES) 
“Principles of Biology”* it is assumed that “increasingly definite dis- 
tinctions among variations are produced wherever there occur definitely- 
distinguished sets of conditions to which the varieties are respectively 
subject,” and only where these occur; for “ Vital actions remain con- 
stant so long as the external actions to which they correspond remain 
constant”; and noreference isanywhere made to the principle that what- 
ever causes sexual separation between dissimilar members of one family, 
race, or species tends not only to perpetuate, but to increase their dis- 
similarity in the succeeding generations. The view maintained in the 
following paper is I believe in better accord with the fundamental prin- 
ciple that ‘‘ Unlike units of an aggregate are sorted into their kinds and 
parted when uniformly subject to the same incident forces,”t as is also 
the teaching of Spencer’s “ Principles of Biology,” in one passage; for I 
have recently discovered that in a single paragraph of this work it is 
maintained that while exposed to the same external conditions, the 
members of the same species may be increasingly differentiated, ‘ until 
at length the divergence of constitutions and modes of life become great 
enough to lead to segregation of the varieties.”¢ If the segregation had 
been introduced as a necessary condition without which the divergence 
of families and races could not take place, the position taken in this 
paragraph would have been essentially the same as the one I have 
adopted. In the next section, however, he abandons the position, using 
the following words: “ Through the process ot differentiation and inte- 
gration, which of necessity brings together, or keeps together, like indi- 
viduals, and separates unlike ones from them, there must nevertheless 
be maintained a tolerably uniform species, so long as there continues a 
tolerably uniform set of conditions in which it may exist. |The italics are 
mine. | 
1 trust my endeavor to contribute something toward the development 
of the theory of divergent evolution will not be attributed to any lack 
of appreciation of what has already been accomplished. The pro- 
pounders of a doctrine which has profoundly influenced every depart- 
ment of modern thought need no praise from me; butas their theory is 
contessedly incomplete, and as one of the leaders in the movement has 
called attention to the need of a re-discussion of the fundamental factors 
of evolution, I offer my suggestions and amendments after prolonged 
and careful study. 
PHYSIOLOGICAL SELECTION AND SEGREGATE FECUNDITY. 
The abstract of Mr. Romanes’s paper on ‘ Physiological selection,” 
given in Nature August 5, 12, and 19, 1886, did not come into my hands 
till the following January, when my theory of divergent evolution 
through cumulative segregation, which had been gradually developing 
*Compare §§ 91, 156, 169, 170 
t See Spencer’s ‘First Principles’, § 166, near the end; also a fuller statement in 
§ 169. 
$See ibid., § 90. 
