Diversity in the Scutes of Chelonia. Yl 



1. External resemblance is not ground for the assumption of 

 reversion (since the presence of two digits in the horse is sometimes 

 due perhaps to the development of the vestigial second digit, or, in 

 other cases, clearly to the duplication of digit three. (Prentiss, '03, 

 p. 299). 



2. Recurrence of a similar abnormality in numerous individuals 

 of the same or different species is not ground for the assumption of 

 reversion. (Cf. duplication of hallux or pollex in man, cat, dog and 

 fowl. Bateson, Prentiss, etc.) 



3. AA^iere extra elements occur which are, presumably, of ves- 

 tigial origin and possibly attributable to reversion, the atavism may 

 be by no means true to the ancestral condition reverted to ; for 

 Prentiss ('03, p. 291) finds that, in a large number of cases, the super- 

 numerary vestigial digit appears in a condition of partial or com- 

 plete duplication — that is to say, in a condition directly misleading 

 if used for inference as to ancestral form. 



4. Supernumerary elements have a negligible value as basis for 

 phylogenetic hypothesis. 



Without regard to Polydactyly, the above considerations suggest 

 themselves a priore against the attribution of definite morphological 

 significance to supernumerary scutes. I refer to Polydactyly because 

 these a priore objections seem to gain weight from their demonstra- 

 bility in another field of variation. 



In regard to scutes the following points may be noted : 



1. We have no evidence as to the value of external resem- 

 blances — as to whether the same appearance might not be due in one 

 individual to one cause, in another to a different cause. 



2. The assumption that anomalies of scutes are atavisms rests 

 on the recurrence of certain more or less characteristic scutes in dif- 

 ferent individuals of the same species, and in different species, and 

 the correspondence of such scutes in a few instances to normal scutes 

 of still other species. But recurrence of certain very definite 

 anomalies in the same or in different species is often noted in cases 

 where atavism w^ould not be suggested. Thus the cyclopean defect 

 occurs in man, and in an essentially similar form in amphibia 

 (Spemann, '04). I have had apparently the same sort of defect to 



