The varieties of Cucurbita pepo which bear large fruit yield a small crop, according to Naudin; 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 yield much milk, and are afterwards capable of fattening well. With fowls which have large top-knots and beards the comb and wattles are generally much reduced in size; though there are exceptions to this rule. 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. Vrolik and Weber (26/9. Prichard 'Phys. Hist. of Mankind' 1851 volume 1 page 324.) 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 (26/10. 'Annales des Sc. Nat.' 1st series tome 19 page 327.) believes that this is determined by the form of the pelvis, which again, no doubt, stands in close relation with their power 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 a direct result of this kind from that consequent on natural selection. Godron has argued (26/11. 'Comptes Rendus' December 1864 page 1039.) that the abortion of the spur on the inner side of the flowers in Corydalis, is caused by the buds at a very early period of growth whilst underground being closely pressed against one another 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. Muller (26/12. "Ueber fotale Rachites" 'Wurzburger Medicin. Zeitschrift' 1860 b. 1 s. 265.) has shown that in shortfaced races of the dog some of the molar teeth are placed in a slightly different position to 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 embryonal cartilages of the bones.
[RELATIVE POSITION OF FLOWERS WITH RESPECT TO THE AXIS, AND OF SEEDS IN THE OVARY, AS INDUCING VARIATION.
In the thirteenth chapter various peloric flowers were described, and their production was shown to be due either to arrested development, or to reversion to a primordial condition. Moquin-Tandon has remarked that the flowers which stand on the summit of the main stem or of a lateral branch are more liable to become peloric than those on the sides (26/13. 'Teratologie Veg.' page 192.); and he adduces, amongst other instances, that of Teucrium campanulatum. In another Labiate plant grown by me, viz., the Galeobdolon luteum, the peloric flowers were always produced on the summit of the stem, where flowers are not usually borne. In Pelargonium, a SINGLE flower in the truss is frequently peloric, and when this occurs I have during several years invariably observed it to be the central flower.