24
ORGANIC EVOLUTION—PHYSICAL
conditions fall within narrow limits. We see, for instance, that though by discipline, aided by selective breeding, one variety of horse has had its locomotive power increased considerably beyond the locomotive powers of other varieties; yet that further increase takes place, if at all, at an unappreciable rate. The different kinds of dogs, too, in which different forms and capacities have been established, do not show aptitudes for diverging in the same directions at considerable rates. In domestic animals generally, certain accessions of intelligence have been produced by culture, but accessions beyond these are inconspicuous. It seems that in each species of organism there is a margin for functional oscillations on all sides of a mean state, and a consequent margin of structural variations; that it is possible rapidly to push functional and structural changes towards the extreme of this margin in any direction, both in an individual and in a race; but that to push these changes further in any direction, and so to alter the organism as to bring its mean state up to the extreme of the margin in that direction, is a comparatively slow process."—Principles of Biology, vol. i. p. 188.[1]
The domesticated dog is presumably descended from one or more of the different wild varieties, or from their relatives the wolves. Now, considering the length of time dogs have been domesticated, and the severity of the selection to which they have been subjected, our largest dogs, the St. Bernards, Newfoundlands, mastiffs, boarhounds, do not very greatly exceed wild dogs or wolves in size, nor do our most intelligent dogs greatly surpass them in intelligence; but our smallest dogs, some of them little bigger than rats, are very much smaller, and some of our tame breeds are exceedingly stupid. Clearly as regards dogs, we have been able to produce little evolution, but great retrogression. In some breeds there has undoubtedly been
