1886 J MR. J. B.SUTTON ON ATAVISM. 551 



then strong on tlie wing, and jiroved, on dissection, to be a male. 

 Those which I put on the reservoirs in our Corporation Park do not 

 appear to have changed in plumage ; there is not as yet the slightest 

 sign of a crest." 



The above rare hybrid has since l)een presented to the British 

 Museum of Natural History. 



The following jjapers were read : — 



1. On Atavism. A Critical and Analytical Study. By 

 J. Bland Sutton^ F.R.C.S.^ Lecturer on Comparative 

 Anatomy, Middlesex Hospital, Erasmus Wilson Lecturer 

 on Pathology, Royal College of Surgeons. 



[Eeceived October 22, 1886.] 



In an interesting paper entitled " Critical Remarks on Polydactyly 

 as Atavism," Gegenbaur enters into a masterly discussion of this 

 confessedly difficult subject, and, in the course of summing-up, he 

 ventures to divide atavistic phenomena into two groups — Paleo- 

 genetic and Neogenetic. 



Atavism he defines as "a re-appearance of a more primitive 

 organization, or a reversion (Riickschlag) to a primary state." To 

 choose an example : — the occasional presence of an os centrale in the 

 adult human carpus is a reversion to a condition very prevalent in 

 the lower Mammalia. We know that a cartilaginous representative 

 of this ossicle is easy of detection in the embryo ; but Atavism does 

 not consist in the existence of a latent germ, but in its becoming 

 perfected and further developed. 



In this case the atavistic part exists, by law of inheritance, in the 

 early embryo as a germ which normally disappears, but in some 

 cases becomes further developed. This is Gegenbaur's Palaeogenetic 

 Atavism. If the abnormal part (using the term aiinormal in its most 

 literal sense) is not found as a germ in the embryo, the reversion is 

 " Neogenetic." 



My object is to show that all examples of atavism belong to the 

 Palaeogenetic group and that Neogenetic Atavism has no existence. 



The question of Polydactyly I do not intend to discuss, but shall 

 select the foot of the Horse, as Gegenbaur has done, to serve as 

 illustrations of the principle, and thence extend the view broadly. 



The descent of the modern Horse from five-toed ancestors is 

 bevond all question. That the animal of to-day walks on an 

 enlarged third digit with a rudimentary digit on each side in the 

 manus and pes is accepted doctrine. The comparative recent ances- 

 tors of the Horse were tridactyle. Gegenbaur states that Hensel's^ 



^ Morph. Jabrbuch. Bd. vi. S. 584-59G. A translation by Drs. Garson and 

 Gadow is given in ' Journal of Anatomy and Physiology,' vol. xvi. p. 015. 



* " Ueber Hipparion mediterraneum." Abk. k. Akad. d. Wissensch. r. 

 Berlin, 1861, S. 66. 



