Few will agree with Colonel H. Smith, who believes that they have descended from no less than five primitive and differently coloured stocks. (2/14. 'Nat. Library, Horses' volume 12 page 208.) But as several species and varieties of the horse existed (2/15. Gervais 'Hist. Nat. Mamm.' tome 2 page 143. Owen 'British Fossil Mammals' page 383.) during the later tertiary periods, and as Rutimeyer found differences in the size and form of the skull in the earliest known domesticated horses (2/16. 'Kenntniss der fossilen Pferde' 1863 s. 131.), we ought not to feel sure that all our breeds are descended from a single species. The savages of North and South America easily reclaim the feral horses, so that there is no improbability in savages in various quarters of the world having domesticated more than one native species or natural race. M. Sanson (2/17. 'Comptes rendus' 1866 page 485 and 'Journal de l'Anat. et de la Phys.' Mai 1868.) thinks that he has proved that two distinct species have been domesticated, one in the East, and one in North Africa; and that these differed in the number of their lumbar vertebra and in various other parts; but M. Sanson seems to believe that osteological characters are subject to very little variation, which is certainly a mistake. At present no aboriginal or truly wild horse is positively known to exist; for it is commonly believed that the wild horses of the East are escaped domestic animals. (2/18. Mr. W.C.L. Martin, 'The Horse' 1845 page 34, in arguing against the belief that the wild Eastern horses are merely feral, has remarked on the improbability of man in ancient times having extirpated a species in a region where it can now exist in numbers.) If therefore our domestic breeds are descended from several species or natural races, all have become extinct in the wild state.
With respect to the causes of the modifications which horses have undergone, the conditions of life seem to produce a considerable direct effect. Mr. D. Forbes, who has had excellent opportunities of comparing the horses of Spain with those of South America, informs me that the horses of Chile, which have lived under nearly the same conditions as their progenitors in Andalusia, remain unaltered, whilst the Pampas horses and the Puno horses are considerably modified. There can be no doubt that horses become greatly reduced in size and altered in appearance by living on mountains and islands; and this apparently is due to want of nutritious or varied food. Every one knows how small and rugged the ponies are on the Northern islands and on the mountains of Europe. Corsica and Sardinia have their native ponies; and there were (2/19. 'Transact. Maryland Academy' volume 1 part 1 page 28.), or still are, on some islands on the coast of Virginia, ponies like those of the Shetland Islands, which are believed to have originated through exposure to unfavourable conditions. The Puno ponies, which inhabit the lofty regions of the Cordillera, are, as I hear from Mr. D. Forbes, strange little creatures, very unlike their Spanish progenitors. Further south, in the Falkland Islands, the offspring of the horses imported in 1764 have already so much deteriorated in size (2/20. Mr. Mackinnon 'The Falkland Islands' page 25. The average height of the Falkland horses is said to be 14 hands 2 inches. See also my 'Journal of Researches.') and strength that they are unfitted for catching wild cattle with the lasso; so that fresh horses have to be brought for this purpose from La Plata at a great expense. The reduced size of the horses bred on both southern and northern islands, and on several mountain-chains, can hardly have been caused by the cold, as a similar reduction has occurred on the Virginian and Mediterranean islands. The horse can withstand intense cold, for wild troops live on the plains of Siberia under lat. 56 deg, (2/21. Pallas 'Act. Acad. St. Petersburgh' 1777 part 2 page 265. With respect to the tarpans scraping away the snow see Col. Hamilton Smith in 'Nat. Lib.' volume 12 page 165.) and aboriginally the horses must have inhabited countries annually covered with snow, for he long retains the instinct of scraping it away to get at the herbage beneath.