70 Eobert E. Coker. 



to the resemblance of a new to an old character; it is a positive 

 assumption of the unbroken continuity of the old quality in latent 

 condition through succeeding generations and its final reappearance 

 as a variation. 



The need for this theory of reversion has been felt asi an explana- 

 tion of the observed fact that the offspring often manifests some 

 character not possessed by its immediate parents, but peculiar to its 

 grandparents or to some more remote ancestors. Especially in the 

 latter case do we, in Darwin's words, "feel a just degree of astonish- 

 ment," and, whether the resemblance is to a near or to a more remote 

 ancestor, does there seem a need for the theory of latent characters 

 and reversion. It would seem, however, that the theory was one to 

 be invoked only in cases where there is strong reason to believe that 

 the resemblance is not superficial or accidental, but so vital and 

 unmistakable as to be explicable only as a direct inheritance. Yet 

 the theory of reversion has been overworked to the point of being 

 applied to cases of the most superficial resemblance, even to varia- 

 tions that bear resemblance only to a purely constructive ancestor, 

 the hypothetical existence of which is based largely on the arbitrary 

 assumption that the anomaly in question is an atavism. 



The subject of polydactylism in higher vertebrates not only offers 

 an excellent illustration of the wholesale use of the theory to explain 

 all kinds of anomaly, but it is one which has been so much longer 

 and more thoroughly studied in its various aspects (anatomy, 

 heredity, etc.) than has the subject of supernumerary scutes, that 

 we may be allowed to draw some lessons of caution from its history, 

 if we do not seem to imply that the same principles must apply to 

 scutes as to digits. 



Darwin himself, "with much hesitation," attributed polydactylism 

 in man to reversion, but soon retracted ("76, p. 459), with the 

 explanation that he had been misled by the statements of observers. 

 The bifid rays of Selachians, and "constructive" Y-toed ancestral 

 mammals, have been called up to account for modern polydactylism 

 (Albrecht, Bardeleben, etc.). Other writers, as Gegenbaur, and, 

 especially, Bateson and Prentiss have put the matter in a better 

 light. Certainly the following simple principles^ self-suggesting, 

 would seem demonstrated in regard to polydactylism. 



