ZOOLOGY AND BOTANY, MICROSCOPY, ETC. 
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{ Natural Selection has been the main, but not the exclusive, means of 
modification.’ 
(4) Even if it were true that all species and all specific characters 
must necessarily owe their origin to Natural Selection, it would still 
remain illogical to define the theory of Natural Selection as indifferently 
a theory of species or a theory of adaptations ; for, even upon this 
erroneous supposition, specific characters and adaptive characters would 
remain very far indeed from being conterminous — most of the more im- 
portant adaptations which occur in Nature being the common property 
of many species. 
(5) In no case can Natural Selection have been the cause of mutual 
infertility between allied, or any other species — i.e. of the most general 
of all £ specific characters.’ 
(6) Without Isolation, or the prevention of free intercrossing, 
organic evolution is in no case possible. Therefore it is Isolation that 
has been ‘ the exclusive means of modification,’ or more correctly, the 
universal condition to it. Therefore, also, heredity and variability 
being given, the whole theory of organic evolution becomes a theory of 
the causes and conditions which lead to isolation. 
(7) Isolation may be either discriminate or indiscriminate. When 
discriminate, it has reference to resemblances between individuals con- 
stituting the isolated colony or group ; when indiscriminate, it has no 
such reference. In the former case there arises Homogamy, and in the 
latter case there arises Apogamy. 
(8) Except where very large populations are concerned, indiscrimi- 
nate isolation always tends to become increasingly discriminate ; and, in 
the measure that it does so, Apogamy passes into Homogamy, by virtue 
of independent variability. 
(9) Natural Selection is one among many other forms of discriminate 
isolation, and presents in this relation the following peculiarities: — 
(a) The isolation is with reference to superiority of fitness ; (6) is effected 
by death of the excluded individuals ; and (c) unless assisted by some 
other form of isolation, can only effect monotypic, as distinguished 
from polytypic, evolution. 
(10) It is a general law of organic evolution that the number of 
possible directions in which divergence may occur can never be more 
than equal to the number of cases of efficient isolation ; but, excepting 
Natural Selection, any one form of isolation need not necessarily require 
the co-operation of another form in order to create an additional form 
of isolation, or to cause polytypic, as distinguished from monotypic, 
evolution. 
(11) Where common areas and polytypic evolution are concerned, 
the most general and most efficient form of isolation has been the phy- 
siological, and this whether the mutual infertility has been the antece- 
dent or the consequent of morphological changes on the part of the 
organisms concerned, and whether or not these changes are of an adap- 
tive character. 
(12) This form of isolation — which, in regard to incipient species, 
I have called physiological selection — may act either alone or in con- 
junction with other forms of isolation on common areas ; in the former 
case its agency is of most importance among plants and the lower 
