3. The duration or permanence of varieties is very different. Some return at once to the normal type when the causes of change are removed. Others perpetuate themselves nearly as invariably as species, and are named races. It is these races only that we are likely to mistake for true species, since here we have that permanent reproduction which is one of the characteristics of the species. The race, however, wants the other characteristics of species as above stated; and it differs essentially in having branched from a primitive species, and in not having an independent origin. It is quite evident that in the absence of historical evidence we must be very likely to err by supposing races to have really originated in distinct "primordial forms." Such error is especially likely to arise if we overlook the fact of the sudden origination of such races, and their great permanency if kept distinct. There are two facts which deserve especial notice, as removing some of the difficulty in such cases. One is that well-marked races usually originate only in domesticated animals, or in wild animals which, owing to accidental circumstances, are placed in abnormal circumstances. Another is, that there always remains a tendency to return, in favorable circumstances, to the original type. This tendency to reversion is much underrated by Darwin and his followers; yet they constantly recur to it as a means of proving possible derivation, and their writings abound in examples of it. Perhaps the most remarkable of these reversions are those which occur when varieties destitute of all the markings of the original stock are crossed and reproduce those markings, which Darwin shows to occur in pigeons and domestic fowls. The domesticated races usually require a certain amount of care to preserve them in a state of purity, both on this account and on account of the readiness with which they intermix with other varieties of the same species. Many very interesting facts in illustration of these points might be adduced. The domesticated hog differs in many important characters from the wild boar. In South America and the West Indies it has returned, in three centuries or less, to its original form.[162] The horse is probably not known in a state originally wild, but it has run wild in America and in Siberia. In the prairies of North America, according to Catlin[163] they still show great varieties of color. The same is the case in Sable Island, off the coast of Nova Scotia[164] where herds of wild horses have existed since an early period in the settlement of America. In South America and Siberia they have assumed a uniform chestnut or bay color. In the plains of Western America they retain the dimensions and vigor of the better breeds of domesticated horses. In Sable Island they have already degenerated to the level of Highland ponies; but in all countries where they have run wild, the elongated and arched head, high shoulders, straight back, and other