VARIATION. 71 



II. IN VERTEBRATES. 

 Class I. VARIATION. 



A. In the number of Digits. 



The number of digits is frequently in excess of the normal 

 number, polydactylism : more rarely fewer are developed ; such 

 specimens have been arranged under the fourth and fifth Classes. 



a. Polydactylus. 



Syn. Perissodactylus, Megalomelus perissodactylus, Ourlt. 

 Several distinct conditions have been arranged under this 

 head*. It has, however, been thought wiser to attempt no 

 division until a much larger collection of specimens can be 

 obtained. Some of these conditions are admitted varieties ; such 

 are the accessory toes of the St. Bernard dog and Dorking fowl : 

 others are probably the effect of fission or dichotomy, either of 

 a single digit, or of several, or even of the entire limb. Some- 

 times the accessory digits resemble adventitious buds, and the 

 supernumerary parts not unfrequently simulate to those of the 

 opposite ]imb. 



294. The foot of a Fowl with the inner toe bearing an accessory 



digit of three phalanges, nearly resembling the second 

 toe of the other foot, on its inner side. The middle toe 

 has also a minute supernumerary nail growing from its 

 last phalanx on the same side. 

 The existence of a supernumerary toe on the inner side of the 



foot is characteristic of the Dorking breed, and is transmitted very 



truly. 



Hunterian. 



295. The skeleton of the other foot of the same bird. 



Hunterian. 



296. The left manus of a Pig, with a small accessory inner toe 



behind the normal one. 



This toe is probably a supernumerary digit added to the nor- 

 mal second digit, and not a thumb, a view amply confirmed by the 

 succeeding preparations. 



Hunterian. 



297. The left maims of a Pig with considerable enlargement of 



* Sec Introduction. 



