3 o6 CAUSES OF VARIATION 



During this time there has been (supposedly) no admixture 

 with domesticated herds, and yet there appears occasionally, 

 even in a Devon herd, a white calf whose ears, lower legs, and 

 brush of tail are marked with the tawny red or brown of the 

 wild ancestor, and whose matted, curly hair, upstanding horns, 

 and peculiar facial expression bespeak his reversion to the 

 early type. 



This singular persistence of characters once typical argues 

 strongly for stability if considered from the standpoint of the 

 ancient character, but it speaks not less plainly for instability if 

 considered from the standpoint of the new (present) type. 



The vermiform appendix, the persistence of the tail in most 

 mammals, these and scores of similar instances attest the 

 stubborn resistance of a structural part to the extinction that is 

 inevitable ; and the case of the " beard " of the turkey cock 

 illustrates the fact that a combination of whatever order, once 

 started, tends strongly to continue, even though useless and 

 unmeaning. 



SECTION IV EVIDENCE FROM DISAPPEARANCE 

 OF PARTS 



The organic world is full of instances of structural parts 

 lingering long after their usefulness has largely or quite disap- 

 peared, and after entirely new relations have been established 

 among associated characters. 1 



The hind legs of the python and the whale, already rudimen- 

 tary and represented only by bones internal to the surface, and 

 those of the sea lion and the seal, evidently disappearing in the 

 same manner; the wing of the apteryx, reduced to the merest 

 trace hidden in the plumage, and that of the ostrich, plainly fol- 

 lowing along to the same fate ; the fetal hair of the whale, and 



1 This is entirely independent of the question of the influence of utility in the 

 origin and development of a new character. It has been the fashion to assume 

 that none but useful characters will originate. The writer, on the contrary, inclines 

 to the belief that any character will arise whose elements are present in the organ- 

 ism, quite irrespective of its usefulness, and that it will continue unless prevented 

 by selection, although manifestly it will never attain maximum prominence except 

 through the cumulative effect of the selective process. 



