Let us briefly sum up the last two chapters on the pigeon. We may conclude with confidence that all the domestic races, notwithstanding their great amount of difference, are descended from the _Columba livia_, including under this name certain wild races. But the differences between these latter forms throw no light whatever on the characters which distinguish the domestic races. In each breed or sub-breed the individual birds are more variable than birds in a state of nature; and occasionally they vary in a sudden and strongly-marked manner. This plasticity of organisation apparently results from changed conditions of life. Disuse has reduced certain parts of the body. Correlation of growth so ties the organisation together, that when one part varies other parts {224} vary at the same time. When several breeds have once been formed, their intercrossing aids the progress of modification, and has even produced new sub-breeds. But as, in the construction of a building, mere stones or bricks are of little avail without the builder's art, so, in the production of new races, selection has been the presiding power. Fanciers can act by selection on excessively slight individual differences, as well as on those greater differences which are called sports. Selection is followed methodically when the fancier tries to improve and modify a breed according to a prefixed standard of excellence; or he acts unmethodically and unconsciously, by merely trying to rear as good birds as he can, without any wish or intention to alter the breed. The progress of selection almost inevitably leads to the neglect and ultimate extinction of the earlier and less improved forms, as well as of many intermediate links in each long line of descent. Thus it has come to pass that most of our present races are so marvellously distinct from each other, and from the aboriginal rock-pigeon. * * * * * {225} CHAPTER VII. FOWLS. BRIEF DESCRIPTIONS OF THE CHIEF BREEDS--ARGUMENTS IN FAVOUR OF THEIR DESCENT FROM SEVERAL SPECIES--ARGUMENTS IN FAVOUR OF ALL THE BREEDS HAVING DESCENDED FROM GALLUS BANKIVA--REVERSION TO THE PARENT-STOCK IN COLOUR--ANALOGOUS VARIATIONS--ANCIENT HISTORY OF THE FOWL--EXTERNAL DIFFERENCES BETWEEN THE SEVERAL BREEDS--EGGS--CHICKENS--SECONDARY SEXUAL CHARACTERS--WING- AND TAIL-FEATHERS, VOICE, DISPOSITION, ETC.--OSTEOLOGICAL DIFFERENCES IN THE SKULL, VERTEBRÆ, ETC.--EFFECTS OF USE AND DISUSE ON CERTAIN PARTS--CORRELATION OF GROWTH.