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Show 34.2 LAW::) OF VARIATION. CHAl'. XXVI. pl'ocess of fusion. The tendency in homologous parts ~o unite during their early development, Moquin-Tandon cons1cl~rs as one of the most striking laws governing the productiOn ?f monsters. It apparently explains a multitude of cases, ?oth m the animal and vegetable kingdoms; it throws a clear bght on many normal structures which have evidently been formed by the union of originally distinct pR,rts, and it possesses, as we shall see in a future chapter, much theoretical interest. On tlte Variability of Multiple and Homologous Parts.Isidore Geoffroy 6 insists that, when any part or organ is repeated many times in the same animal, it is particularly liable to vary both in number and structure. vVith respect to number, the proposition may, I think, be considered as fully established; but the evidence is chiefly derived from organic beings living under their natural conditions: with which we are not here concerned. "When the vertebrre, or teeth, or rays in the fins of fishes, or feathers in the tails of birds, or petals, stamens, pistils, and seeds in plants, are very numerous, the number is generally variable. The explanation of this simple fact is by no means obvious. With respect to the variability in structure of multiple parts, the evidence is not so decisive; but the fact, as far as it may be trusted, probably depends on multiple parts being of less physiological importance than single parts; consequently their perfect standard of structure has been less rigorout:;ly enforced by natural selection. Compensation of Growth, or Balancernent.-This law, as applied to natural species, was propounded by Goethe and Geoffroy St. Hilaire at nearly the same time. It implies that, when much organised matter is used in building up some one part, other parts are starved and become reduced. Several authors, especially botanists, believe in this law; others reject it. As far as I can judge, it occasionally holds good; but its importance has probably been exaggerated. It is scarcely possible to distinguish between the supposed effects of such compensation of growth, and the effects of long-continued selection, which 6 'Hi~t. des Anomalies,· tom. iii. pp. 4, 5, G. CHAP. XXVI. COMPENSATION. 343 may at the same time lead to the augmentation of one part and the diminution of another. There can be no doubt that an organ may be greatly increased without any corresponding diminution in the adjoining parts. To recur to our former illustration of the Irish elk, it may be asked what part has suffered in consequence of the immense development of the horns? It has already been observed that the struggle for existence does not bear hard on our domesticated productions ; conse· quently the principle of economy of growth will seldom affect them, and we ought not to expect to find frequent evidence of compensation. We have, however, some such cases. :MoquinTandon describes a monstrous bean/ in which the stipules were enormously developed, and the leaflets apparently in consequence completely aborted; this case is interesting, as it represents the natural condition of Lathyrus aphaca, with its stipules of great size, and its leaves reduced to mere threads, which act as tendrils. De Oandolle 8 has remarked that the varieties of Raphanus sativus which have small roots yield numerous seed, valuable from containing oil, whilst those with large roots are not productive in this latter respect; and so it is with B1·a,.ssica asperifolia. The varieties of the potato which produce tubers very early in the season rarely bear flowers; but Andrew Knight,9 by checking the growth of the tubers, forced the plants to flower. The varieties of Oucurbita pepo which produce large fruit yield, according to N audin, few in number; whilst those producing small fruit yield a vast number. Lastly, I have endeavoured to show in the eighteenth chapter that with many cultivated plants unnatural treatment checks the full and proper action of the reproductive organs, and they are thus rendered more or less sterile; consequently, in the way of compensation, the fruit becomes greatly enlarged, and, in double flowers, the petals are greatly increased in number. With animals, it has been found difficult to produce cows which should first yield much milk, and afterwards be capable of 7 ' Teratologie V eg.,' p. 156. See abo my paper on climbing plants in 'Joul'nal of Linn. Soc. Bot.,' vol. ix. 1 1865, p. 114. s ':M:emoil'cs du 1\'Iuseum,' &c., tom. viii. p. 178. 9 Loudou's 'Encyclop. of Gardening,' p. 829. |