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Show 1886.] MR. J. B. SUTTON ON ATAVISM. 551 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 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. B L A N D S U T T O N , F.R.C.S., Lecturer on Comparative Anatomy, Middlesex Hospital, Erasmus Wilson Lecturer on Pathology, Royal College of Surgeons. [Received 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-PALEOG E N E T I C and N E O G E N E T I C. 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. W e 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 abnormal in its most literal sense) is not found as a germ in the embryo, the reversion is " Neogenetic." M y 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 tbe 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 ancestors of the Horse were tridactyle. Gegenbaur states that Hensel's2 1 Morph. Jabrbucb. Bd. vi. S. 584-596. A translation by Drs. Garson and Gadow is given in ' Journal of Anatomy and Physiology,' vol. xvi. p. 615. 2 " Ueber Hipparion mediterraneum." Abk. k. Akad. d. Wissensch. z. Berlin, 1861, S. m. |