GENE UAL INTRODUCTION. XXXV 



or dicliotom}-. But if the extra digit lias not in this case 

 been split off from the lai'ge middle digit, its existence 

 cnn only be accounted for in one of two ways. It is 

 either a sport — an instance of abrupt or discontinuous 

 variation — or a restoration of one of the digits found in 

 the three-toed ancestors. Seeing that the horse embryo 

 starts with three toes, and that the lateral digits persist 

 in a nearly perfect though minute form for several months, 

 it seems to me more natural to account for the occasional 

 presence of an asymmetrical extra digit, such as occurs in 

 some of my specimens, by the reversion theory than by 

 any other. As it happens, in the inner (second) digit of 

 the embryo horse I especially examined some years ago 

 the first and second phalanges had all but united. Had 

 this embryonic digit been favoured from the first by an 

 unusual amount of nourishment, it might have increased 

 to form a large extra digit instead of degenerating to 

 form a mere vestige (the ''button") at the end of the 

 inner "splint'^ bone. If in some cases the extra digits 

 in the horse correspond to digits which persisted in the 

 horse family well through the tertiary period, these digits 

 form a very striking instance of reversion. 



(c) Forearm. — Keeping to the fore-limb, still another 

 case of reversion may be mentioned. In the ancestors of 

 the horse, as in ourselves, there were two distinct bones 

 in the forearm, the radius and ulna. In all the works 

 dealing with the skeleton of the horse the uhia is de- 

 scribed as incomplete. It is said to terminate in a slender 

 process some distance from the lower end of the radius^ 

 In very young horse embryos I find the tihia is not only 

 as complete, but nearly as large as the roAius. But it is 

 often complete, though not entirely ossified, in foals, and 

 occasionally complete in the adult. In the skeleton of 

 the horse placed by Sir William Flower in the entrance 

 hall of the Natural History Museum, London, there is a 

 complete ulna, and in all horses the lower end of the ulna 

 persists and enters freely into the wrist ('* knee ") joint. 

 In having at times a complete ulna in the horse we have 

 another instance of reversion. 



