56 THE EVOLUTION THEORY 



domestic pigeon in the still living rock-dove (ColuTiiha llviu), the 

 ancestral form common to the horse and the ass is extinct, and we 

 can only suppose that it was striped like a zebra, because such striping 

 occasionally occurs in pure horses and pure asses, at least in their 

 youth, although now only on the legs, and because this striping is 

 •often more marked in the hybrid between the horse and the ass, the 

 mule. In Italy, where one sees hundreds of mules, the striping is not 

 •exactly frequent, but it may occur in about two per cent., while in 

 America it is said to be much more frequent. The germ-plasm of the 

 horse and the ass must therefore contain, in varying numbers, ids 

 whose skin-colour determinants represent in part still unmodified 

 ancestral characters. When two germ-cells chance to meet in fer- 

 tilization, both of which have received, through a favourable reducing 

 division, a relatively large number of such ids, a relative majority of 

 these in the fertilized ovum is opposed to the dissimilar and therefore 

 mutually neutralizing homologous determinants of horse and ass, and 

 reversion to the ancestral form occurs. 



These cases of reversion are enough to show us that the old 

 unmodified ancestral determinants may persist in the germ-plasm 

 through long series of generations. But an even deeper glimpse into 

 the dim ancient history of our modern species of horses is afibrded by 

 the occurrence of three-toed horses, references to a small number of 

 which the palaeontologist Marsh was able to discover in literature, 

 ^nd one of which he was able to observe in life. Julius Caesar 

 possessed a horse whose three-toed feet represented a reversion to the 

 horses of Tertiary times, Mesohiiypus, MioJdppus, and Protohippus or 

 Hqypai'ion ; for all these genera possessed, in addition to the strong 

 middle toe, two weaker and shorter lateral toes. 



In the germ-plasm of our modern horses there must still persist 

 in certain ids the determinants of the ancestral foot, which, after a long 

 succession of favourable reducing divisions combined with favourable 

 chances of fertilization, may come to be in a majority, and may thus 

 be able to induce a reappearance of a character which has long been 

 hidden under the surface of the present-day type of the species. 



I do not propose to enter further on the discussion of the 

 phenomena of inheritance. A more detailed investigation of the 

 phenomena of reversion is to be found in ' The Germ-plasm' published 

 ten years ago ; and the discussion could not be resumed here without 

 a critical consideration of a relatively large series of newly acquired 

 facts not always harmonizing, and, as yet, not even fully available. 

 The year 1900 has given us the investigations of three botanists, 

 De Vries, Correns, and Tschermak, who have sought by experiments 



