REGRESSION 89 



which have regressed towards a more ancient order of things ; 

 and at every stage of development structures are absent which 

 were present in the life-history, because they were then 

 useful, but which since underwent complete regression. 

 Doubtless if a higher animal, a man for instance, lived during 

 his development in a succession of environments similar to 

 those in which his race was evolved, his development would 

 much more exactly recapitulate the life-history than it actually 

 does ; for in that case structures which had been useful 

 during the life-history would continue to be so during the 

 development, and so would be preserved. 



150. Instances of this are common enough in nature. Thus 

 a butterfly, in its development from the egg forwards, lives, 

 during part of its development, in environments which must 

 correspond pretty closely to the environments in which its 

 race underwent this or that part of the evolution. The 

 caterpillar, therefore, must represent its prototype in the 

 corresponding stage of the life-history much more closely 

 than does the human embryo at any stage. The caterpillar 

 stage is therefore greatly prolonged, as compared to the other 

 stages, and, during it, the animal probably very closely 

 resembles its remote ancestor. 1 But during the other stages, 

 in which the environment differs greatly from the ancient 

 environments for instance, the stage during which the ovum 

 develops into the caterpillar, and in the chrysalis stage when 

 the caterpillar develops into the butterfly the environments 

 differ greatly from those which anciently prevailed. In these 

 stages, therefore, much regression is possible, for many parts 

 become useless which were formerly useful. The develop- 

 ment is then only a vague recapitulation of the life-history, 

 and consequently is much shortened and condensed. In 

 these respects it resembles the whole development of man. 



151. Consider how vastly different is the environment, in 

 which the embryo of man develops, from the environments 

 in which his race evolved. The embryo develops in the 

 uterus, but its prototypes of the life- history struggled each 

 for itself in a world full of enemies and dangers. How many 

 parts, therefore, have become useless to the embryo, which 

 were useful to the prototypes ! How vast is the field in 



1 This is no doubt true in a general sense. But Lord Avebury has 

 shown that larvae of insects belonging to very different orders may be 

 closely similar if of similar habits, but that they may be very unlike, 

 even when belonging to the same order, if their habits and therefore 

 environments be dissimilar. In the latter case progressive evolution has 

 occurred in the larval stage. 



