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 aérial—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 Reversed 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- 
