With the increased or diminished length of the beak the length of the slit forming the external orifice of the nostrils varies, but not in due proportion, for, taking the rock-pigeon as the standard, the orifice in a short-faced tumbler was not shortened in due proportion with its very short beak. On the other hand (and this could not have been anticipated), the orifice in three English carriers, in the Bagadotten carrier, and in a runt (_pigeon cygne_), was longer by above the tenth of an inch than would follow from the length of the beak proportionally with that of the rock-pigeon. In one carrier the orifice of the nostrils was thrice as long as in the rock-pigeon, though in body and length of beak this bird was not nearly double the size of the { 169} rock-pigeon. This greatly increased length of the orifice of the nostrils seems to stand partly in correlation with the enlargement of the wattled skin 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, the eyelids, measured longitudinally, are proportionally more than double the length of those of the rock-pigeon.
The great difference (see woodcut No. 27) in the curvature of the 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 maxillo-jugal arch with the premaxillary bones. But in carriers, runts, and barbs the singular reflexion of the upper margin of the middle part of the lower jaw (see woodcut No. 25) is not strictly correlated with the width or divergence (as may be clearly seen in woodcut No. 26) of the premaxillary bones, but with the breadth of the horny and soft parts of the upper mandible, which are always overlapped by 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 the seventh pair furnished with processes; the sacral and caudal vertebræ 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 the rock-pigeon. In fantails, the length and number of the caudal vertebræ have increased. Hence, during the gradual progress of variation and selection, the internal bony frame-work and the external shape of the body have been, to a certain extent, modified in a correlated manner.