62 ADAPTATION AND PROGRESS 
selection, for Darwin continues: “There can also be little 
doubt that the tendency to vary in the same manner has 
often been so strong that all the individuals of the same species 
have been similarly modified without the aid of any form of 
selection.” 
As to conditions favorable for the production of new forms 
through natural selection, Darwin mentions a large and diverse 
area, numbers of individuals in the species, intercrossing (espe- 
cially among hermaphrodites) and isolation.1 Diversification 
of structure is considered an adaptive quality under some cir- 
cumstances and is discussed at length,? and origin of species is 
accounted for as the cumulative result of ever increasing diversi- 
fications which in time become fixed.* 
In considering the degree to which organization tends to 
advance, Darwin discusses the question of standards of judging 
advancement and accepts that of Von Baer, namely, ‘“‘ the 
amount of differentiation of the parts of the same organic being, 
(in the adult state, Darwin adds) . . . and their specialization 
for different functions . . . or the completeness of the division 
of physiological labor.” 
Not only does natural selection lead to the origin of new 
species, but also to the extinction of intermediate forms.’ ‘‘ Use 
and disuse of organs” is linked with natural selection, so also 
“acclimatization,” ‘correlated variation,” and ‘compensation 
and economy of growth ”’; § then follows a frank discussion of the 
difficulties in the way of accepting his theory. ‘‘ Some of them,” 
he says, ‘‘ are so serious that to this day I can hardly reflect on 
them without being in some degree staggered.”’ 7 
1 Origin of Species, pp. 81 ff. This last element, first stressed by Wagner, was 
given great prominence by Romanes and more recently by David Starr Jordan. “In 
the principle of isolation,’ says Romanes, ‘** we have a principle so fundamental and 
so universal, that even the great principle of natural selection lies less deep, and 
pervades a region of smaller extent. Equalled only in its importance by the two 
basal principles of heredity and variation, this principle of isolation constitutes 
the third pillar of a tripod on which is reared the whole superstructure of organic 
evolution.” — Darwin and after Darwin, ii, p. 2. 
2 Origin of Species, pp. 86 ff. 5 Tbid., pp. 59, 93, 134f. 
3 Ibid., pp. go fi. 6 Tbid., ch. V. 
4 Ibid., p. 97. 7 [bid., p. 133. 
