Organic Evolution. 2 1 7 



of the caudal vertebra?. 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 con- 

 nection 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 corre- 

 lated ; 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. 

 In the freedom of nature such superficial variations are 

 not so obvious. But among the invertebrates they are not 

 inconsiderable. The case of land-snails, already quoted, 

 may again be cited. Taking variations in banding alone, 

 Mr. Cockerell knows of 252 varieties of Helix nemoralis 

 and 128 of H. hortensis. Still, among the wild relatives of 

 our domestic breeds of animals and birds the superficial 

 variations are decidedly less marked. And this is partly 

 due to the fact that they are in a state of far more stable 

 equilibrium than our domestic products, and partly to the 

 constant elimination of all variants which are thereby 

 placed at a serious or vital disadvantage. White rats, 

 mice, or small birds, in temperate regions, would soon be 

 seized upon by hawks and other enemies. If the eggs and 

 young of the Kentish plover, shown in our frontispiece, 

 were white or yellowish, like the eggs and young of our 

 fowls, they would soon be snapped up. The varied protec- 

 tive resemblances, general and special, have been brought 

 about by the superficial variations of organisms, and the 

 elimination of those which, from non-variation or wrong 

 variation, remained conspicuous. We need only further 

 notice one thing here, namely, that, in the case of special 



