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Show 324: LAWS OF VARIATION. CHAP. XXV. to diminish the feet, the beak becoming at the sa~o tim.e through correlation shorter; but that in those few breeds m w~ICh len~th of beak has been a selected point, the feet, notw1thstandmg disuse have through correlation increased in size. Wi~h the increased length of the beak in pigeon~, not only the tongue increases in length, but likewise the onfice of t~e nostrils. But the increased length of the orifice of the nostnls perhaps stands in closer correlation with the development of the corrugated skin or wattle at the base of ~he beak; for when there is much wattle round the eyes, the eyelids are greatly increased or even doubled in length. There is apparently some correlation ev~n in colour between the head and the extremities. Thus with horses a .large white star or blaze on the forehead is generally accompamed by white feet.s With white rabbits and cattle, dark marks often co-exist on the tips of the ears and on the feet. In black and tan dogs of different breeds, tan-coloured spots over the eyes and tan-coloured feet almost invariably go together. These latter cases of connected colouring may be due either to reversion or to analogous variation,-subjects ~o which :ve shall her~after return,-·but this does not necessanly determme the questiOn of their original correlation. If those naturalis~s are c?rrect who maintain that the jaw-bones are homologous with the hmb-bones, then we can understand why the head and limbs tend to vary together in shape and even in colour; but several highly competent judges dispute the correctness of this view. The lopping forwards and downwards of the immense ears of fancy rabbits is in part due to the disuse of the muscles, and in part to the weight and length of the ears, which have been increased by selection during many generations. Now, with the increased size and changed direction of the ears, not only has the bony auditory meatus become changed in outline, direction, and greatly in size, but the whole skull has been slightly modified. This could be clearly seen in" half-lops"that is, in rabbits with one ear alone lopping forward-for the opposite sides of their skulls were not strictly symmetrical. This seems to me a curious instance of correlation, between hard a 'The Farrier and Naturalist,' vol. i., 1828, p. 456. 1 CHAP. XXV. CORRELATED VARIABILITY. 325 bones and organs so soft and flexible, as well as so unimportant under a physiological point of view, as the external ears. The result no doubt is largely due to mere mechanical action, that is, to the weight of the ears, on the same principle that the skull of a human infant is easily modified by pressure. The skin and the appendages of hair, feathers, hoofs, horns, and teeth, are homologous over the whole body. Every one knows that the colour of the skin and that of the hair usually vary together ; so that Virgil advises the shepherd to look whether the mouth and tongue of the ram are black, lest the lambs should not be purely white. With poultry and certain ducks we have seen that the colour of the plumage stands in some connexion with the colour of the shell of the egg,-that is, with the mucous membrane which secretes the shell. The colour of the skin and hair, and the odour emitted by the glands of the skin, are said 9 to be connected, even in the same race of men. Generally the hair varies in the same way all over the body in length, fineness, and curliness. The same rule holds good with feathers, as we see with the laced and frizzled breeds both of fowls and pigeons. In the common cock the feathers on the neck and loins are always of a particular shape, called hackles: now in the Polish breed, both sexes are characterised by a tuft of feathers on the bead; but through correlation these feathers in the male always assume the form of hackles. The wing and tail-feathers, though arising from parts not homologous, vary in length together; so that long or short winged pigeons generally have long or short tails. The case of the J acobin-pigeon is more curious, for the wing and tail feathers are remarkably long ; and this apparently has arisen in correlation with the elongated and reversed feathers on the back of the neck, which form the hood. The hoofs and hair are homologous appendages ; and a careful observer, namely Azara, 10 states that in Paraguay horses of various colours are often born with their hair curled and twisted like that on the head of a negro. This peculiarity is strongly inherited. But what is remarkable is that the hoofs of these horses "are " absolutely like those of a mule." The hair also of the mane and tail is invariably much shorter than usual, being only from four 9 Godron, 'Sur l'Espcco,' tom. ii. p. 217. 10 'Quadrupedes clu Paraguay,' tom. ii, p. 333. |
