Compensation of Growth, or Balancement.—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
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; consequently 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. Moquin-Tandon describes a monstrous bean,[[849]] 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 Candolle[[850]] 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 Brassica asperifolia. The varieties of the potato which produce tubers very early in the season rarely bear flowers; but Andrew Knight,[[851]] by checking the growth of the tubers, forced the plants to flower. The varieties of Cucurbita pepo which produce large fruit yield, according to Naudin, 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
fattening well. With fowls which have large topknots and beards the comb and wattles are generally much reduced in size. Perhaps the entire absence of the oil-gland in fantail pigeons may be connected with the great development of their tails.
Mechanical Pressure as a Cause of Modifications.—In some few cases there is reason to believe that mere mechanical pressure has affected certain structures. Every one knows that savages alter the shape of their infants' skulls by pressure at an early age; but there is no reason to believe that the result is ever inherited. Nevertheless Vrolik and Weber[[852]] maintain that the shape of the human head is influenced by the shape of the mother's pelvis. The kidneys in different birds differ much in form, and St. Ange[[853]] believes that this is determined by the form of the pelvis, which again, no doubt, stands in close relation with their various habits of locomotion. In snakes, the viscera are curiously displaced, in comparison with their position in other vertebrates; and this has been attributed by some authors to the elongation of their bodies; but here, as in so many previous cases, it is impossible to disentangle any direct result of this kind from that consequent on natural selection. Godron has argued[[854]] that the normal abortion of the spur on the inner side of the flower in Corydalis, is caused by the buds being closely pressed at a very early period of growth, whilst under ground, against each other and against the stem. Some botanists believe that the singular difference in the shape both of the seed and corolla, in the interior and exterior florets in certain compositous and umbelliferous plants, is due to the pressure to which the inner florets are subjected; but this conclusion is doubtful.
The facts just given do not relate to domesticated productions, and therefore do not strictly concern us. But here is a more appropriate case: H. Müller[[855]] has shown that in
short-faced races of the dog some of the molar teeth are placed in a slightly different position from that which they occupy in other dogs, especially in those having elongated muzzles; and as he remarks, any inherited change in the arrangement of the teeth deserves notice, considering their classificatory importance. This difference in position is due to the shortening of certain facial bones, and the consequent want of space; and the shortening results from a peculiar and abnormal state of the basal cartilages of the bones.
Relative Position of Flowers with respect to the Axis, and of Seeds in the Capsule, as inducing Variation.