[34] ‘Le Pigeon Voyageur Belge,’ 1865, p. 87. I have given in my ‘Descent of Man’ (6th edit. p. 466) some curious cases, on the authority of Mr. Tegetmeier, of silver-coloured (_i.e._ very pale blue) birds being generally females, and of the ease with which a race thus characterised could be produced. Bonizzi (_see_ ‘Variazioni dei Columbi domestici:’ Padova, 1873) states that certain coloured spots are often different in the two sexes, and the certain tints are commoner in females than in male pigeons. [35] Prof. A. Newton (‘Proc. Zoolog. Soc.,’ 1865, p. 716) remarks that he knows no species which present any remarkable sexual distinction; but Mr. Wallace informs me, that in the sub-family of the Treronidæ the sexes often differ considerably in colour. _See also_ on sexual differences in the Columbidæ, Gould, ‘Handbook to the Birds of Australia,’ vol. ii., pp. 109-149. [36] I am not sure that I have designated the different kinds of vertebræ correctly: but I observe that different anatomists follow in this respect different rules, and, as I use the same terms in the comparison of all the skeletons, this, I hope, will not signify. [37] J. M. Eaton’s ‘Treatise,’ edit. 1858, p. 78. [38] In an analogous, but converse, manner, certain natural groups of the Columbidæ, from being more terrestrial in their habits than other allied groups, have larger feet. _See_ Prince Bonaparte’s ‘Coup d’œil sur l’Ordre des Pigeons.’ [39] It perhaps deserves notice that besides these five birds two of the eight were Barbs, which, as I have shown, must be classed in the same group with the long-beaked Carriers and Runts. Barbs may properly be called short-beaked Carriers. It would, therefore, appear as if, during the reduction of their beaks, their wings had retained a little of that excess of length which is characteristic of their nearest relations and progenitors. [40] Temminck, ‘Hist. Nat. Gén. des Pigeons et des Gallinacés,’ tom. i., 1813, p. 170. [41] This term was used by John Hunter for such differences in structure between the males and females, as are not directly connected with the act of reproduction, as the tail of the peacock, the horns of deer, etc. CHAPTER VI. PIGEONS—_continued._ ON THE ABORIGINAL PARENT-STOCK OF THE SEVERAL DOMESTIC RACES—HABITS OF LIFE—WILD RACES OF THE ROCK-PIGEON—Dovecot-PIGEONS—PROOFS OF THE DESCENT OF THE SEVERAL RACES FROM COLUMBA LIVIA—FERTILITY OF THE RACES WHEN CROSSED—REVERSION TO THE PLUMAGE OF THE WILD ROCK-PIGEON—CIRCUMSTANCES FAVOURABLE TO THE FORMATION OF THE RACES—ANTIQUITY AND HISTORY OF THE PRINCIPAL RACES—MANNER OF THEIR FORMATION—SELECTION—UNCONSCIOUS SELECTION—CARE TAKEN BY FANCIERS IN SELECTING THEIR BIRDS—SLIGHTLY DIFFERENT STRAINS GRADUALLY CHANGE INTO WELL-MARKED BREEDS—EXTINCTION OF INTERMEDIATE FORMS—CERTAIN BREEDS REMAIN PERMANENT, WHILST OTHERS CHANGE—SUMMARY.