88 THE PRINCIPLES OF HEREDITY 



progressive variations lapse, and the character vanishes 

 never to reappear, except in the improbable event of a fresh 

 series of matings and variations 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 whose habits have ceased 

 to be aerial was evolved by the superimposition in a long 

 line of individuals of progressive variation on progressive 

 variation. These, when the character becomes useless, are 

 lapsed in orderly succession, the most modern first and more 

 ancient later, 1 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 life-history exactly; for, during the whole course of evolu- 

 tion, reversion was at work, planing away everything which 

 was originally useless, or which became useless as the organ 

 evolved. A complex organ such as a wing is, therefore, even 

 during its progression, a product, not only of progression, but 

 also of regression. Evolution rough-hews the organ, but re- 

 version chisels its finer lines. What is true of a complex 

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

 and plant. Such a being is a product, not only of progressive 

 evolution, but also of regressive evolution. In it many 

 structures, useful during a remote period of the life-history, 

 but useless later, have returned to that yet more ancient 

 condition when they did not exist. Others, in which re- 

 version is as yet incomplete, still persist, and are known to 

 us as vestigial remains. It should, however, be noted clearly 

 that, when a vestigial structure is more developed earlier in 

 the development of the individual than later, this indicates 

 that its apparent regression is due not wholly to regression 

 the result of reversion, but partly or wholly to progression the 

 result of reversed selection. Such a structure must have 

 become not only useless, but harmful during the life-history. 



149. Every complex animal, therefore, in the successive 

 stages of its development does not represent exactly the cor- 

 responding successive stages in the evolution of the race. At 

 each stage of the development are present useless structures, 



1 By this it is meant, of course, only that variations which were super- 

 imposed on one another are lapsed in orderly succession. It is not meant 

 that characters in different parts of the wing necessarily disappear in the 

 reverse order of their appearance. Doubtless such characters do often 

 disappear in this order, but they do not necessarily do so. 



