1898] A THEORY OF RETROGRESSION 403 



accentuated by Natural Selection. The useless variations, the vast 

 majority, are planed away by reversion. Most of them being minute, 

 disappear in the next generation, but, even when they are com- 

 paratively great, a very few generations 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 char- 

 acter 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 proto- 

 types 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 that yet more ancient con- 

 dition when they had not come into existence. Others, in which 

 reversion is yet incomplete, still persist, and are known to us as 

 vestigial remains. It should, however, be noted that, when a 

 vestigial structure is more developed earlier in the ontogeny than 

 it is later, this indicates that its retrogression is due not only 

 to reversion the result of true atavism, but to false reversion the 

 result of Re versed Selection. Such a structure must have become 

 not merely useless, but worse than useless during the phylogeny. 



Every complex animal, therefore, in the successive stages of its 

 development does not represent exactly successive stages in the 

 evolution of its race. At each stage of the ontogeny are present 

 useless structures, or useless parts of structures, which have retro- 



