[F6] That the conditions are now and again very trying in Shetland is proved by the death-rate among native sheep being, in some districts, from 40 to 50 per cent. during the winter of 1912–13.
[F7] ‘Cyclopædia of American Agriculture,’ vol. iii. p. 34.
[F8] Walker’s ‘History of the Hebrides,’ vol. ii. 1808.
Evidence in support of the view that the descendants of the Spanish and south-country horses, introduced about 1710 and after 1745, were soon either eliminated or dwarfed, we have from Dr Johnson and others. Dr Johnson tells us that the pony he rode in Coll was very low, and that a bulky man on one of the native horses made “a very disproportionate appearance,” and, after referring to the small horses of Rum, mentions that he had heard of a yet smaller race in Barra, “of which the highest is not above 36 inches.” Barra was one of the islands which benefited by the introduction of Spanish horses by Clanranald.
That dwarfing may be the direct result of an inadequate supply of nourishment during development is suggested by the condition of fœtuses I found some years ago in a wild rabbit. The right uterine horn of this rabbit contained four young, the left eight. The eight in the left horn were as well developed as the four in the right horn, but only half the size and half the weight. Evidently the amount of nourishment available in this case was limited, and as the eight in the left horn only received as much as the four in the right horn, they were only half the size.
But while dwarfing may be, or appear to be, directly caused by the environment, there are other possible causes. Sometimes the small size of one or more members of a family or litter is due to reversion to small ancestors. For example, in a litter of five puppies bred some months ago (from parents in which West Highland and Mexican (Spanish?) blood prevails) there is marked variation in size. When these pups of mixed origin were weighed three days after birth, two males weighed 7·5 oz. each, two females 5·5 oz. each, and one male 4·5 oz. When again weighed at the end of the twelfth week, the largest male scaled 106 oz., the smallest male 44 oz., the larger female 58 oz., and the smaller female 46 oz. In this case the small male reproduced his small Mexican great-grandsire, while the large males took after their West Highland ancestors.
But dwarfs often enough turn up in old-established “pure” breeds, and now and again a dwarf is found in an otherwise normal human family. There is no reason for supposing that such dwarfs are the result of reversion. Just as one of a litter of pups may prove a dwarf, one of five or six foals, full brothers and sisters, may prove a dwarf.
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.