1886. ] MR. J. B) SUTTON ON ATAVISM. 5oL 
then strong on the wing, and proved, on dissection, to be a male. 
Those which I put on the reservoirs in our Corporation Park do not 
appear to have changed i in plumage ; there is not as yet the slightest 
sign of a crest.’ 
The above rare hybrid has since been presented to the British 
Museum of Natural History. 
The following papers were read :— 
1. On Atavism. A Critical and Analytical Study. By 
J. Buanp Surton, F.R.C.S., Lecturer on Comparative 
Anatomy, Middlesex Hospital, Erasmus Wilson Lecturer 
on Pathology, Royal College of Surgeons. 
[Received October 22, 1886.] 
In aninteresting 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—PaL«£o- 
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 Palzogenetic 
Atavism. If the abnormal part (using the term abnormal 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 
Palzeogenetic 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 
beyond 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. 8. 584-596, A translation by Drs. Garson and 
Gadow is given in ‘ Journal of An: atomy and Physiology,’ vol. xvi. p. 615. 
2 “Ueber Hipparion mediterraneum.” Abk. k. Akad. d. Wissensch. z. 
Berlin, 1861, S. 66. 
