It may, I think, be assumed that in the case of horses living under natural conditions, “spontaneous variation,” without the aid of reversion, will, as a rule, provide sufficient material to admit of a variety being evolved well adapted in size and other respects for the conditions which at the moment prevail. It need hardly be pointed out that little will be gained by speculating as to whether dwarfing is due to the direct influence of the environment, to reversion, or to spontaneous variation. Many breeders, more especially breeders of dogs, seem to think that dwarfing is invariably the result of inbreeding. It is doubtless true that the members of many recently formed pigmy breeds are closely inbred, but it is well to bear in mind that there are closely inbred large as well as closely inbred small breeds—that size is largely a matter of selection—that in the case of natural races size depends more on the surroundings and the extent of the range than on the consanguinity. The view that dwarfing is caused by inbreeding is insisted on by Sir Everett Millais in a book on Rational Breeding. Sir Everett states that, though in the case of the Shetland pony the “climate, bad food, &c., had been a factor in reducing the size, the primary cause was inbreeding due to isolation.” It seems to me, however, highly probable that until artificial selection began in earnest about thirty years ago, inbreeding had little influence in determining the size of Shetland ponies—that isolation had been a decidedly more potent factor than inbreeding. Scottish red deer are decidedly smaller now than they were in Roman times; but this is not so much due to inbreeding as to the range of most of the herds being restricted, and to the best stags being cut off before they have a chance of improving the herd. The Scottish deer in New Zealand, though all descended from a few imported individuals, instead of dwindling in size, are larger and carry finer heads than their home-bred relatives,—the wider range and better conditions have more than compensated for the inevitable in-and-in breeding. It is doubtless true that in-and-in breeding sooner or later diminishes the vigour, size, and fertility, and, in addition, restricts variation. But under natural conditions, if the range is sufficiently extensive, occasional reversion to vigorous ancestors will prevent dwarfing provided there is rigid elimination of the unfit. If Shetland ponies are the pigmy descendants of one or more ancient races at least as tall as Exmoor and Welsh ponies, one would expect them to increase in size when bred and reared under favourable conditions. It has been again and again asserted that “the climate and comparative privation of the Shetland Isles were necessary to maintain the small stature of the ponies, and that the breed would inevitably lose this and all other characteristics if bred away from Shetland and under more generous conditions.”