Different breeds of pigeons.—The various breeds of domestic pigeons furnish one of the best examples of the changes in structure which breeders can bring about, after several generations, by careful crossing. In spite of the great differences between, say, the [Pg 280] tumbler, with its habit of turning head-over-heels in the air; the pouter, with its exaggerated crop; the Jacobin, with its neck-feathers reversed; the fantail, with its large number of erect tail-feathers; and others,—there can be no doubt that all these varieties have been artificially produced from the rock-pigeon; and it is curious to observe occasional reversions to the characters of the ancestral stock. “The rock-pigeon is of a slaty blue, with white loins.... The tail has a terminal dark bar, with the outer feathers externally edged at the base with white. The wings have two black bars. Some semi-domestic breeds, and some truly wild breeds, have, besides the two black bars, the wings chequered with black. These several marks do not occur together in any other species of the whole family. Now, in every one of the domestic breeds, taking thoroughly well-bred birds, all the above marks, even to the white edging of the outer tail-feathers, sometimes concur perfectly developed. Moreover, when two birds belonging to two or more distinct breeds are crossed, none of which are blue or have any of the above-specified marks, the mongrel offspring are very apt suddenly to acquire these characters.”[16] This is a striking instance of the tendency exhibited by living things to revert to ancestral characters, even though these latter may have lain dormant for hundreds of generations. EXERCISES ON CHAPTER XV. 1. Why does a bird require a long neck, and a keel upon the breast bone? (1898) 2. Draw the bones of a bird’s wing, and mark the places of insertion of the largest quills. (1898) 3. What bones carry the primary and secondary quills of a bird’s wing, and the quills of the tail? Illustrate your answer by a drawing. (1901) [Pg 281] 4. Point out the chief differences between the skulls of a quadruped and a bird. (1901) 5. Describe one of the large feathers of a bird’s wing. (1898) 6. Draw general plans of the fore-limbs of a mammal and a bird. Letter the principal bones, so as to show how they correspond in the two animals. (1901) 7. If the skull of a bird were placed before you, what features would enable you to recognise it with certainty? (1901) 8. How does the fact that a bird stands on two legs affect the skeleton of the trunk? (1897) Fig. 184a.—Comparison of Bat and Bird. 9. Point out the chief differences between the wing of a bird and that of a bat. 10. Explain how the barbs of a quill feather are attached to one another. Describe a feather at the time when it is forcing its way through the skin. (1905)