THE MODES OF THE FOUR PRINCIPLES. 81 
under /, regressive selection, due to the survival of variations pre- 
viously excluded; and in its indiscriminate aspects under z, indis- 
criminate elimination. So also election, and isolation, and partition 
has each its reflexive mode, produced by the action of the members 
of the species upon each other, and its environal mode, determined 
by the relations between the environment and the species; also its 
regressive aspects, caused by the cessation or reversal of the influence 
that has been ruling, and its indiscriminate aspects. The letters (7, 
k, l, 2) here used in designating the different forms correspond with 
those used in the tables given in Chapter VIII. 
4. The Reflexive Mode of Influence. 
The forms of reflexive selection have been more fully worked out 
than have those of reflexive isolation, or reflexive election, or reflexive 
partition. Of the forms of reflexive selection, sexual selection is 
the most familiar; for Darwin discussed its effects on the evolution 
of the higher animals and especially emphasized its importance in 
producing the different races of man. It may be found that some 
of the effects which he attributed to this principle are produced 
in other ways; but there can be no doubt that in the evolution of 
mankind it is a factor of the greatest importance. With the advance 
of civilization the action of natural selection is checked; but the result 
is not as disastrous as it otherwise would be but for the increasing 
stringency of sexual, social, and institutional selection in preventing 
the marriage of those who are most deficient. Darwin recognized 
that the forms of sexual selection may not only change without any 
change in the environment surrounding the species, and without 
securing any advantage for the species in its relations to the environ- 
ment, but that it may even establish a standard of selection that is 
somewhat at variance with the standard maintained by natural selec- 
tion, and that it may in such cases be the deciding influence, causing the 
species to lose certain characters which are at the time of the change of 
some advantage in its relations to the environment. This he thought 
must have been the case when the ancestors of the human race first lost 
their covering of hair. (See Descent of Man, Chap. XX.) The em- 
phasis that Darwin laid on the action of sexual selection in securing 
the coérdination between the sexual instincts of either sex and the 
instincts and palpable qualities of the other sex has gradually led to 
the recognition of certain other codrdinations between members of 
the same race, which must be secured by other forms of reflexive selec- 
tion. These other forms are like sexual selection, in that they are 
subject to change without change in the environment of the species. 
