84 ZEBRAS, HORSES, AND HYBRIDS 



ancestor; thus there has been much arguing in and 

 about a vicious circle. But the fact that a terra has 

 been illogically apphed does not destroy the existence 

 of that which the term signifies, and there can be no 

 doubt that reversion exists. That it exists in the 

 Equidae is shown by the following proofs : (i) The 

 ancestors of the horse had four premolar teeth in the 

 upper jaw; the modern horse has lost, or is losing, the 

 first of these, and as a rule has only three. When the 

 first is present — the so-called wolf-tooth — it is small, 

 and soon disappears. Zebras usually retain the 

 ancestral number. A few years ago Professor Ewart 

 had a Shetland pony in which the first premolar was 

 relatively nearly as large as it is in hipparion, one of 

 the supposed ancestors of the horse. (2) There is no 

 doubt that the horse is descended through three-toed 

 ancestors from five-toed ancestors. All trace of the 

 latter condition is now lost in development, but an 

 embryo horse six weeks old has three toes as cora- 

 pletely forraed as those of a rhinoceros. The outer 

 toes then begin to dwindle, and the newly-born foal 

 supports itself on its central digit alone ; but horses 

 are occasionally born with two digits, each encased in 

 a hoof, and at very rare intervals with three. Caesar's 

 favourite horse was polydactylous, and so was Alex- 

 ander's Bucephalus. Major Waddell, in his book on 

 the Himalayas, refers to a creamy fawn - coloured 

 pony, which ' had a black stripe down the spine . . . 

 broad black stripes over the shoulders, flanks, and 

 legs, and dappled spots over the haunches.' Many 

 other instances might be quoted, but enough has been 

 said to show that reversion is found in the Equidas, as 

 in other families of animals. 



We now pass to the experiments made at Penycuik 

 in crossing the zebra Matopo with various mares of 

 different breeds. 



I. Matopo was first mated with Mulatto, one of Lord 



