THE ROLE OF RETROGRESSION IN EVOLUTION 119 



parts, as their functions and anatomical relations changed, have been 

 so greatly and continually modified that, like a much-mended 

 garment, the original structures have nearly disappeared and have 

 been replaced or almost replaced by new material. Others again 

 have been so much less modified that the careful observer is able 

 to gather a good deal concerning the life-history. In yet others, 

 generally the more modern structures, for example the antlers of 

 the stag, the life-history is plain : only variations which were never 

 incorporated into the life-history of the race have vanished ; the 

 structures develop in the individual on much the same lines as they 

 were evolved in the race. In every species, therefore, we find 

 evidence of an immense amount of retrogression. Consider, for 

 example, how much man must have lost since he became a verte- 

 brate, and how much more in the even longer antecedent period. 



196. Retrogression, then, has been as much a part of evolution, 

 of adaptation, as progression. It has planed away useless and 

 burdensome structures. It has straightened, simplified, and 

 abbreviated recapitulation ; the life-history is not told in all its 

 details ; the narrative is limited to particulars essential to its 

 development ; and thus development is rendered possible during 

 the brief lifetime of the individual. If a man, for example, 

 recapitulated all the changes undergone by his ancestors, he would 

 require, not years, but ages for development. Lastly, retrogression, 

 combined with some progression, has rendered possible the exist- 

 ence of the developing individual in environments vastly different 

 from those inhabited by its prototypes of the ancestry. The 

 human embryo, for instance, dwells within its mother's uterus. Its 

 prototypes had an independent existence. Were they, with their 

 structures and faculties complete, placed in the uterus, they would 

 perish just as surely as the embryo would were it placed in the 

 ancestral environments. 



197. Within the uterus the embryo leads a protected vegetative 

 life. It has no such active struggle for existence as the adult. 

 The now useless power, which depends on structures and faculties, 

 of fighting actively for itself, as did its prototypes, who lived in a 

 world where nutriment was scarce and enemies plentiful, has been 

 lost through retrogression. The sole business of the embryo is to fit 

 itself as quickly and as thoroughly by development as its nature per- 

 mits for the battle which occurs later. Quick development implies 

 quick recapitulation, which in turn implies abbreviated recapitula- 

 tion. In all the higher animals the length of the period of gestation 

 is always proportionate to, and therefore presumably dependent on, 



