The Variation of Animals and Plants Under Domestication, Vol. I.

by Charles Darwin

	Rock Pigeon.	Pouter, from Mr. Bult.	Tumbler, Dutch Roller.	Bussorah Carrier.
Cervical Vertebræ	12	12	12	12 The 12th bore a small rib.
Dorsal Vertebræ	8	8	8	8
" Ribs	8 The 6th pair with processes, the 7th pair without a process.	8 The 6th & 7th pair with processes.	7 The 6th & 7th pair without processes.	7 The 6th & 7th pair without processes.
Sacral Vertebræ	12	14	11	11
Caudal Vertebræ	7	8 or 9	7	7
Total Vertebræ	39	42 or 43	38	38

The pelvis differs very little in any breed. The anterior margin of the ilium, however, is sometimes a little more equally rounded on both sides than in the rock-pigeon, The ischium is also frequently rather more elongated. The obturator-notch is sometimes, as in many tumblers, less developed than in the rock-pigeon. The ridges on the ilium are very prominent in most runts.

In the bones of the extremities I could detect no difference, except in their proportional lengths; for instance, the metatarsus in a pouter was 1.65 inch, and in a short-faced tumbler only .95 in length; and this is a greater difference than would naturally follow from their differently-sized bodies; but long legs in the pouter, and small feet in the tumbler, are selected points. In some pouters the scapula is rather straighter, and in some tumblers it is straighter, with the apex less elongated, than in the rock-pigeon: in the woodcut, fig. 28, the scapulæ of the rock-pigeon (A), and of a short-faced tumbler (B), are given. The processes at the summit of the coracoid, which receive the extremities of the furcula, form a more perfect cavity in some tumblers than in the rock-pigeon: in pouters these processes are larger and differently shaped, and the exterior angle of the extremity of the coracoid, which is articulated to the sternum, is squarer.

Fig. 29.—Furculæ, of natural size. A. Short-faced Tumbler B and C. Fantails. D. Pouter.

The two arms of the furcula in pouters diverge less, proportionally to their length, than in the rock-pigeon; and the symphysis is more solid and pointed. In fantails the degree of divergence of the two arms varies in a remarkable mariner. In fig. 29, B and C represent the furculæ of two fantails; and it will be seen that the divergence in B is rather less even than in the furcula of the short-faced, small-sized tumbler (A); whereas the divergence in C equals that in a rock-pigeon, or in the pouter (D), though the latter is a much larger bird. The extremities of the furcula, where articulated to the coracoids, vary considerably in outline.

In the sternum the differences in form are slight, except in the size and outline of the perforations, which, both in the larger and lesser sized breeds, are sometimes small. These perforations, also, are sometimes either nearly circular, or elongated, as is often the case with carriers. The posterior perforations occasionally are not complete, being left open posteriorly. The marginal apophyses forming the anterior perforations vary greatly in development. The degree of convexity of the posterior part of the sternum differs much, being sometimes almost perfectly flat. The manubrium is rather more prominent in some individuals than in others, and the pore immediately under it varies greatly in size.

Correlation of Growth.—By this term I mean that the whole organisation is so connected, that when one part varies, other

parts vary; but which of two correlated variations ought to be looked at as the cause and which as the effect, or whether both result from some common cause, we can seldom or never tell. The point of interest for us is that, when fanciers, by the continued selection of slight variations, have largely modified one part, they often unintentionally 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 short-faced tumbler, both of which have very short beaks, the tongue, taking the rock-pigeon as the standard of comparison, was proportionally 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 the beak from the tip to the feathered base was exactly thrice as long as in a first-rate short-faced tumbler, the tongue was only a little more than twice as long. But the tongue varies in length independently of the beak: thus, in a carrier with a beak 1.2 inch in length, the tongue was .67 in length; whilst in a runt which equalled the carrier in length of body and in stretch of wings from tip to tip, the beak was .92 whilst the tongue was .73 of an inch in length, so that the tongue was actually longer than in the 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 the other.

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

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.

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. This is well seen in jacobins, and still more plainly in runts, some varieties of which have their wings and tail of great length, whilst others have both very short. With jacobins, the remarkable length of the tail and

wing-feathers is not a character which is intentionally selected by fanciers; but fanciers have been trying for centuries, at least since the year 1600, to increase the length of the reversed feathers on the neck, so that the hood may more completely enclose the head; and it may be suspected that the increased length of the wing and tail-feathers stands in correlation with the increased length of the neck-feathers. Short-faced tumblers have short wings in nearly due proportion with the reduced size of their bodies; but it is remarkable, seeing that the number of the primary wing-feathers is a constant character in most birds, that these tumblers generally have only nine instead of ten primaries. I have myself observed this in eight birds; and the Original Columbarian Society[^[311]] reduced the standard for bald-head tumblers from ten to nine white flight-feathers, thinking it unfair that a bird which had only nine feathers should be disqualified for a prize because it had not ten white flight-feathers. On the other hand, in carriers and runts, which have large bodies and long wings, eleven primary feathers have occasionally been observed.