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Show 414 DARWINISM CHAP. thus been increased (or diminished) by selection, there is in the offspring a strong tendency to re:vert to a mean or ~verago size, which tends to check further mcroa.se. . ~nd. th~s .mean appears to be, not the mean of the .actual ex1stmg md1v1dual:; but a lower mea.n, or that from whiCh they had been recentlr raised by selection. I He calls this the law of "Rew·ession towards Mediocrity," and it has been proved by expenments with vegetables and by observations on mankind. This regression, in every generation, takes place even when both parents have been selected for their high development of the organ in ·question ; but when there is no such selection, and crosses arc allowed amonO' individuals of every grade of development, the deterioration ~vill be very rapid ; and after a time not only will the avemo-e size of the part be greatly reduced, but the instances of f~ll development will become very rare. Thus what \Voismann terms "panmixia," or free intercrossing, will co-operate with Galton's law of "regression t?wards mediocrity," and the result will be that, whenever scloctwn ceases to act on any part or organ which ~as heretofore be~n kept ?-P to. a maximum of size and efficiency, tho organ m qucstwn w1ll rapidly decrease till it reaches a mean value considerably below the mean of the progeny that has usua.lly been produced each year, and very greatly below tho mean of that portion which has survived annually; and this will take place by tho general law of heredity, and quite irrespective of any 11se or disuse of the part in question. Now, no observations have been adduced by Mr. Spencer or others, showing that the average amount of chango supposed to be due to disuse is greater than that due to the law of regression towards mediocrity ; while even if it were somewhat greater, we can see many possible contributory causes to its production. In the case of civilised man's diminished jaw, there may well be some correlation between the jaw and the brain, seeing that increased mental activity would lead to the withdrawal of blood and of nervous energy from adjacent parts, and might thus lead to diminished growth of those parts in the individual. And in ~he. c~se of pet-dogs, the selection of small or s.hort-headed md~VIduals would imply the unconscious selectwn of those With less massive temporal muscles, and thus lead to the concomitant 1 Joumal of the Anthropological Institute, vol. xv. pp. 246-260. XIV FUNDAMENTAL PROBLEMS 415 rc<.lu<.:tion of those muscles. Tho amount of reduction observed Ly Darwin in the wing-bones of domestic ducks and poultry, and in tho hind legs of tame rabbits, is very small, anu is certainly no greater than tho above causes will well account for ; while so many of tho external characters of all our domestic animals have been subject to long-continued artificial selection, and we arc so ignorant of the possible correlations of different parts, that the phenomena presented by them seem sufficiently explained without recurrence to the assumption that any changes in the individual, due to disuse, arc inherited by the offi pring. Supposed Ejj'ects of Disuse arnnng JVild A11imals. It may be urged, however, that among wild animals we have many undoubted results of disuse much more pronounced than tho e among domestic kinds, results which cannot be explained by the causes already adduced. Such arc the reduced size of the wings of many birds on oceanic islands ; tho abortion of the eyes in many cave animal., a.ncl in some which live underground ; and the loss of tho hind limbs in whales and in some lizards. These cases differ greatly in the amount of the reduction of parts which has taken place, and may be duo to different causes. It is remarkable that in some of tho birds <>f oceanic islands the reduction is little if any greater than in domestic birds, as in the wa.ter-hen of Tristan d'Acunha. Now if the reduction of wing were duo to the hereditary effects of disuse, we should expect a very much greater effect in a bird inhabiting an oceanic island than in a domestic bird, whore the disuse has been in action for an indefinitely shorter period. In tho case of many other birds, however-as some of the New Zealand rails and the extinct dodo of Mauritius-the wings have been reduced to a much more rudimentary condition, though it is still obvious that they were once organs of flight ; and in these cases we certainly require some other causes than those which have reduced the wings of our domestic fowls. One such cause may have been of the same natnrc as that which has been so efficient in reducing the wings of the insects of oceanic islands--the destruction of those which, during the occasional use of their wings, wore carried out to ~ea. This form of natural selection may well have acted in the case of |