APPENDIX E 251 



only here and there is a variation useful. The useful variations, 

 in proportion to their usefulness, are preserved, and, in succeed- 

 ing generations, are accentuated by Natural Selection. The use- 

 less variation, the vast majority, are planed away by reversion. 

 Most of them being minute, disappear in the next generation, 

 but, even when they are comparatively great, a very few genera- 

 tions suffice to procure their disappearance. Even should a 

 series of individuals happen to vary in such a manner that, in 

 each successive individual, a useless character is more and more 

 accentuated, yet, since the tendency towards atavism is greater 

 than towards evolution, a time surely comes when, perhaps in a 

 single generation, the whole of the evolutionary variations lapse, 

 and the character vanishes, never to reappear, except in the 

 improbable event of fresh evolution of a like nature. Again, it 

 sometimes happens that a change of environment renders useless 

 a structure which was formerly useful. Here also reversion steps 

 in and procures its elimination. Such a structure — say the wing 

 of a bird, the habits of which have ceased to be aerial — was 

 evolved by the superimposition in a long line of individuals of 

 favourable variation on favourable variation. These, when the 

 character becomes useless, are lapsed in orderly succession, the 

 most recent first, the more ancient later ; till, at last, the structure 

 reverts to that most ancient condition when it did not exist. In 

 this manner it approximates continually to more and more ancient 

 forms, but only approximates. It never reproduces its prototypes 

 of the phylogeny exactly, for, during the whole course of evolution, 

 reversion was at work, planing away everything which was 

 originally useless, or which became useless as the environment 

 changed. A complex organ such as a wing is, therefore, a 

 product not only of evolution, but also of reversion. Evolution 

 rough-hews the organ, but reversion chisels its finer lines. What 

 is true of a complex organ is true in a yet greater degree of every 

 complex plant and animal. Such a being is a product not only 

 of evolution, but also of reversion. In it many structures useful 

 during a remote period of the phylogeny, but useless later, have 

 disappeared utterly by reversion to the yet more ancient condition 



