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, 2 and origin of species is 

 accounted for as the cumulative result of ever increasing diversi- 

 fications which in time become fixed. 3 



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." 4 



Not only does natural selection lead to the origin of new 

 species, but also to the extinction of intermediate forms. 5 " Use 

 and disuse of organs " is linked with natural selection, so also 

 "acclimatization," "correlated variation," and "compensation 

 and economy of growth "; 6 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 5. 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. 6 Ibid., pp. 59, 93, 134 f. 



3 Ibid., pp. 90 ff. Ibid., ch. V. 



4 Ibid., p. 97. 7 Ibid., p. 133. 



