Our domesticated animals exemplify very fully the superficial variations which, through man's selection, have in many cases been segregated and to some extent stereotyped. It is unnecessary to do more than allude to the variations in form and coloration of dogs, cattle, fowls, and pigeons. These variations are not _necessarily_ in any way correlated with any deeper organic variations. They are, however, in many cases so correlated. For example, the form of the pouter pigeon is correlated with the increased size of the crop, the length of the beak carries with it a modification of the tongue, the widely expanded tail of the fantail carries with it an increase in the size and number of the caudal vertebræ. And here we might take the whole series of secondary sexual characters. These and their like may be said to be direct correlations. But there are also correlations which are seemingly indirect, their connection being apparently remote. That in pigeons the size of the feet should vary with the size of the beak; that the length of the wing and tail feathers should be correlated; that the nakedness of the young should vary with the future colour of the plumage; that white dogs should be subject to distemper, and white fowls to the "gapes;" that white cats with blue eyes should be nearly always deaf;--in these cases the correlation is indirect. But from the existence of correlation, whether direct or indirect, it follows that variations seldom come singly. The organism is so completely a unity that the variation of one part, even in superficial matters, affects directly or indirectly other parts.