1 84 THE VARIATION OF ANIMALS IN NATURE 



any special adaptive qualifications. Recent studies have much 

 diminished the value of Darwin's subsidiary hypotheses. 

 Consequently the lack of any clear demonstration that naturally 

 occurring varieties do indeed experience a differential mortality 

 is all the more serious. Tschulock (1922, p. 290) calls ' The 

 Origin of Species ' ' ein logisches Monstrum,' because it 

 deals with the secondary issue before the primary. It seems 

 to us to deserve this censure far more because it fails to 

 demonstrate the actual occurrence of the process which it 

 seeks to establish as the cause of evolution. 



2. Subsequent Confirmation and Development of the 

 Theory. — It is pertinent to inquire whether the theory has 

 undergone any radical modification as a result of the enlarge- 

 ment of the field of inquiry, and whether it needs to be restated 

 in a form different from that presented by Darwin. 



It seems to us that the theory has persisted in very much 

 the same form as that in which it was originally presented. 

 There is no need to enlarge on the fact that Darwin's belief 

 in the heritable effect of ' changed conditions ' was abandoned 

 by most students under the influence of Weismann's teaching. 

 Although we do not suggest that the evidence in favour of 

 the environmental origin of mutations impels us to return to 

 Darwin's somewhat vague and naive belief in the importance 

 of ' changed conditions,' we think that it cannot be sum- 

 marily dismissed, and that more allowance has to be made 

 for the likelihood that mutations may be due to external 

 causes. There are, however, two points on which modern 

 investigation compels us to revise the conception of selection 

 itself. 



(1) Fisher (1930, chapter i) has very clearly shown the 

 effect on the concept of selection of the discovery that in- 

 heritance is governed by a particulate instead of the blending 

 principle which Darwin — perhaps against his better judgment 

 (cf. Fisher, I.e. pp. 1-4) — had in mind. The point at issue 

 is that, with a blending principle at work, ' if not safeguarded 

 by intense marital correlation, the heritable variance is 

 approximately halved in every generation,' and ' to maintain 

 a stationary variance fresh mutations must be available in 

 each generation to supply the half of the variance so lost.' 

 On the particulate theory the mutation-rate may be far 

 smaller than that required by the blending principle. 



