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Show 168 DOMESTIC PIGEONS. CIIAP. V. parts vary; but which of two correlated variations ought to Le looked at as the cause and which as the effeet, or whether both result Jrom some common cnuse, we can seldom or never tell. 'l'ho point of intere t for us is that, when fanciers, by .the continued selection of slio-ht variations, have largely modified one pnrt, they often unin~entionally produce other modifications. For instance, the beak is readily acted on by selection, and, with its increased or diminished length, the tongue increases or diminishes, but not in due proportion; for, in a barb and shortlaced tumbler, both of which have very short beaks, tho tongue, taking the rock-pigeon as tho standard of comparison, was pro-. portionally not shortened enough, whilst in two carriers and in a runt the tongue, proportionally with the beak, was not lengthened enough. Thus, in a first-rate English carrier, in which tho beak from tho tip to the feathered base was exactly thrice as long as in a first-rate short-faced tumbler, tho tongue was only a little more than twice as long. But tho tongue varies in ]ongth independently of tho beak: thus, in a carrier with a ucak 1· 2 inch in length; tho tongue was . 67 in length ; whilst iu a runt whi ·h quailed the can·ier in length of body and in stretch of wings from tip to tip, the beak was · 92 whilst the iOJ1guo was · 73 of an inch in length, so that the tongue was actually longer than in tho carrier with its long beak. The tongue of the runt was also very broad at the root. Of two runts, one had its beak longer by · 23 of an inch, whilst its tongue was shorter by · 14 than in tho other. With tho increased or diminished length of tho beak the length of tho slit forming tho external orifice of the nostrils varies, but not in due proportion, for, taking tho rock-pigeon as th standard, tho orifice in a short-faced tumbler was not shortened in duo proportion with its very short beak. On tho other hand (and this could not have boon anticipated), the orifice in three English carriers, in the Bagadotten carrier, and in a runt (pigeon cygne), was longer by above tho tenth of an inch than would follow from tho length of the beak proportionally with that of tho rock-pigeon. In one carrier tho orifice of the nostrils was thrice as long as in tho rock-pigeon, though in body and length of beak this Lird was not nearly double the size of the , ' CllkP. v. CORRELATION OF GROWTII. 169 rock-pigeon. This greatly increased length of the orifice of the nostrils scorns to stand partly in correlation with the enlargement of tho wattled kin on the upper mandible and over the nostrils ; and this is a character which is selected by fanciers. So again, the broad, naked, and wattled skin round the eyes of carriers and barbs is a selected character; and in obvious correlation with this, tho eyelids, measured longitudinally, are proportionally more than double the length of those of the rock-pigeon. Tho great difference (see woodcut No. 27) in the curvature of tho lower jaw in the rock-pigeon, the tumbler, and Bagadotten carrier, stands in obvious relation to the curvature of the upper jaw, and more especially to the angle formed by the maxillojugal arch with the premaxillary bones. But in carriers, runts, and barbs the singular refl.exion ofthe upper margin of the middle part of tho lower jaw (sec woodcut No. 25) is not strictly correlated with the width or divergence (as may be clearly seen in woodcut No. 26) of the premaxiHary bones, but with the breadth of tho horny and soft parts of tho upper mandible, which arc always overlapped Ly the edges of the lower mandible. In pouters, the elongation of the body is a selected character, and the ribs, as we have seen, have generally become very broad, with tho seventh pair furnished with processes; the sacral and caudal vertobrm have been augmented in number ; the sternum has likewise increased in length (but not in the depth of the crest) by · 4 of an inch more than would follow from the greater bulk of the body in comparison with that of tho rock-pigeon. In fantails, the length and number of the caudaL vertebrm have increased. Hence, during the gradual progress of variation and selection, tho internal bony frame-work and tho external shape of the body have been, to a certain extent, modified in a correlated manner. Although the wings and tail often vary in length independently of each other, it is scarcely possible to doubt that they generally tend to become elongated or shortened in correlation. 'l'his is well seen in jacobins, and still more plainly in runts, some varieties of which have their wings aml tail of great length, whilst others have both very short. With jacobins, the remarkable length of the tail and |