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Show 322 IJA WS OF VARIATION. CHAP. XXV. to the surprise of agriculturists, the ancient rules for judging the age of an animal by the state of its teeth are no longer trustworthy.3 Correlated Variation of Homologous Parts.-Parts which are homologous tend to vary in the same manner; and this is what might have been expected, for such parts are identical in form and structure during an early period of embryonic development, and are exposed in the egg or womb to similar conditions. The symmetry, in most kinds of animals, of the corresponding or homologous organs on the right and left sides of t~e body,. is the simplest case in point; but this symmetry sometimes fails, as with rabbits having only one ear, or stags with one horn, or with many-horned sheep which sometimes carry an additional horn on one side of their heads. With flowers which have regular corollas, the petals generally vary in the same manner, as we see in the same complicated and elegant pattern, on the flowers of the Chinese pink ; but with irregular :flowers, though the petals are of course homologous, this symmetry often fails, as with the varieties of the Antirrhinum or snapdragon, or that variety of the kidney-bean (Phaseolus multifiorus) which has a white standard-petal. In the vertebrata the front and hind limbs are homologous, and they tend to vary in the same manner, as we see in long and short-legged, or in thick and thin-legged races of the horse and dog. I8idore Geoffroy 4 has remarked on the tendency of supernumerary digits in man to appear, not only on the right and left sides, but on the upper and lower extremities. Meckel has insisted 5 that, when the muscles of the arm depart in number or arrangement from their proper type, they almost always imitate those of the leg; and so conversely the varying muscles of the leg imitate the normal muscles of the arm. In several distinct breeds of the pigeon and fowl, the legs and the two outer toes are heavily feathered, so that in the trumpeter pigeon they appear like little wings. In the featherlegged bantam the " boots" or feathers, which grow from the outside of the leg and generally from the two outer toes, have, 3 Prof. J. B. Simonds, on the Age of the Ox, Sheep, &c., quoted in ' Gard. Chronicle,' 1854, p. 588. 4 'Hist. des Anomalies,' tom. i. p. 674. 5 Quoted by Isid. Geoffroy, idem, tom. i. p. 635. CHAP. XXV. CORRELATED VARIABILITY. 323 according to the excellent authority of Mr. Hewitt/ been seen to exceed the wing-feathers in length, and in one case were actually nine and a half inches in length! As Mr. Blyth has remarked to me, these leg-feathers resemble the primary wingfeathers, and are totally unlike the fine down which naturally ?Tows on the legs of some birds, such as grouse and owls. Hence It may be suspected that excess of food has first given redunda~ cy. to the plumage, and then that the law of homologous vana~1~n has led to the development of feathers on the legs, in a po~ItiOn corresponding with those on the wing, namely, on the outside of the tarsi and toes. I am strengthened in this belief ~y the following curious case of correlation, which for a long hme seemed to me utterly inexplicable, namely, that in pigeons of any breed, if the legs are feathered, the two outer toes are p~rtially co~nected by skin. These two outer toes correspond with our third .and fourth toes. Now, in the wing of the pigeon or any other. bird, ~he first and fifth digits are wholly aborted; the second IS rudimentary and carries the so-called " bastardwing;" whilst the th.ird and fourth digits are completely united a~d enclosed b! skm, together forming the extremity of the WI~g. So that m. feather-footed pigeons, not only does the ex-teriOr surface support a row of long feathers, like wing-feathers but t~e very same cli.gits which in the wing are completely united by skm become partially united by skin in the feet; and thus by the law of the corre~ated variation of homologous parts we can understand the curiOus connection of feathered legs and membrane between the two outer toes. . Andrew Knight 7 has remarked that the face or head and the limbs vary together in general proportions. Compare for instance the head .and limbs of a dray and race-horse, or of~ greyhound and mastiff. What a monster a greyhound would appear with t~e head of a mastiff! The modern bulldog, however, has fine hmbs, but this. is a. recently-selected character. From the ~easurements given m the sixth chapter, we clearly see that ~~all the breeds of the pigeon the length of the beak and the SIZ~ of the feet are correlated. The view which, as before explamed, seems the most probable is, that disuse in all cases tends ; 'The Poultry Book,' by W. B. Tegetmeier, 1866, p. 250. A. Walker on Intermarriage, 1838, p. 160. y 2 |
