xm PHYLUM CHORDATA 367 



1. EXAMPLE OF THE CLASS. THE COMMON PIGEON (Columba 



lima, var. domestica) . 



The Common or Domestic Pigeon is known under many varieties, 

 which differ from one another in size, proportions, coloration, 

 details in the arrangements of the feathers, and in many points of 

 internal anatomy. The Pouters, Carriers, Fantails, and Tumblers 

 may be mentioned as illustrating extreme forms. All these varieties 

 have, however, been produced by artificial selection, that is, by 

 breeders selecting, generation after generation, the Birds which 

 most nearly attained to some artificial standard of perfection, 

 breeding from them alone, and killing off the inferior strains. The 

 ancestral species from which the domestic breeds have in this way 

 been evolved is the Rock Pigeon (Columba livia), which is widely 

 distributed in the Palasarctic and Oriental regions. The following 

 description refers especially to the common Dovecot Pigeon. 



External Characters. In the entire Bird (Fig. 1033) the 

 plump trunk appears to be continued insensibly into the small, 

 mobile head, with its rounded brain-case and prominent beak, 

 formed of upper and lower jaws covered by horny sheaths. The 

 head, neck, and trunk are invested in a close covering of feathers, 

 all directed backwards and overlapping one another. Posteriorly 

 the trunk gives origin to a number of outstanding feathers which 

 constitute what is ordinarily called the tail. From the anterior 

 region of the trunk spring the wings, also covered with feathers, 

 and, in the position of rest, folded against the sides of the body. 

 The legs spring from the hinder end of the trunk, but, owing to 

 the thick covering of feathers, only the feet are to be seen in the 

 living Bird, each covered with scales and terminating in four digits 

 (dg. 1' dg. 4'), three directed forwards and one backwards. 



In order to make a fair comparison of the outer form with that 

 of other Craniate types, it is necessary to remove the feathers. 

 When this is done, the Bird is seen to have a long, cylindrical, and 

 very mobile neck, sharply separated both from head and trunk. 

 The true tail is a short, conical projection of the trunk, known as the 

 uropygium, and giving origin to the group of large feathers (ret.) to 

 which the word "tail" is usually applied. On the dorsal surface 

 of the uropygium is a papilla bearing on its summit the opening 

 of a large gland, the oil-gland (o.gl.), the secretion of which is used 

 for lubricating or " preening " the feathers. 



The wings show the three typical divisions of the fore-limb, 

 upper arm, fore-arm, and hand, but the parts of the hand are 

 closely bound together by skin, and only three imperfectly-marked 

 digits, the second (dg. 2) much larger than the first (dg. 1) and 

 third (dg. 3), can be distinguished. In the position of rest the three 

 divisions of the wing are bent upon one another in the form of a Z ; 

 during flight they are straightened out and extended so that the 



