90 THE PRINCIPLES OF HEREDITY 



which regression has worked ! Is it any wonder that the 

 development of man is only a vague recapitulation of his life- 

 history? In plants, which, since they are stationary, live 

 under practically the same conditions from embryonic to 

 adult life, the development of the individual has been even 

 more emended and condensed. It presents, therefore, scarcely 

 a trace of the life-history. 



152. Regression, then, is the necessary complement of pro- 

 gressive evolution. Without regression due to reversion there 

 could be no evolution, except of the simplest kind. Without 

 it there could be no planing away of the numberless useless 

 variations which occur during, and especially at the end of 

 the development, nor of all those structures which, though 

 useful in some part of the life-history, became useless later. 

 Without regression, therefore, a species would become so 

 burdened with useless variation and structures as to be 

 incapable of existence. Reversed selection could not cause 

 the elimination of all these burdensome and useless char- 

 acters; for no matter how burdensome, and worse than 

 useless, they might be in the aggregate, separately they are 

 so little burdensome that reversed selection could not act. It 

 could not act on them in the aggregate, for this would mean 

 that in some individuals they would be present en masse, 

 whereas they would be absent en masse in others a conjunc- 

 tion improbable, or rather impossible. Moreover, reversed 

 selection usually causes a lengthening, not a shortening of 

 the development. 



153. Again, without regression, the recapitulation of the 

 life-history would be impossible in the development ; and for 

 this reason, once again, evolution would be impossible. For 

 were there no regression, the prototypes of the life-history, 

 with all their useless variations, would necessarily be repro- 

 duced exactly in the development of the individual ; and then 

 the development would be as elaborate, and almost as lengthy 

 as regards time, as evolution. Moreover, the prototypes could 

 not exist in the enormously changed environment. How, for 

 instance, could a gill-breathing animal, or any of the higher 

 forms which intervened between it and man, exist in the 

 uterus, in which can exist only those dim representations of 

 the life-history which constitute man's development ? 



154. It is this great change of environment, this close 

 protection of the individual in the uterus and afterwards, that 

 has rendered possible the evolution of man and the higher 

 animals. Opportunity has been afforded for regression to 

 plane away innumerable characters that had become useless. 



