112 HEREDITARY CHARACTERS 



by evolution of the more highly differentiated animals 

 would have been impossible. If between the single cell 

 stage the fertilised ovum and the adult, the human 

 organism had to recapitulate exactly the life-history of all 

 its ancestors, the period of time would be impossibly long. 

 Organs and characters which were useful to the remote 

 ancestors, would be not only useless but actually injurious 

 to the human individual when passing through the various 

 stages of its life-history. The gills, useful to the free- 

 swimming aquatic animal in enabling it to breathe, would 

 be useless to the human foetus which derives its oxygen 

 from the maternal blood. Regressive variation has there- 

 fore eliminated or abbreviated many stages in the life- 

 history. Besides elimination, however, additions have also 

 been made. Progressive as well as regressive variations 

 have occurred at every stage. Thus the mechanism for 

 supplying oxygen to the human embryo has been developed, 

 by progressive variations, concurrently with the elimination 

 of the functional gills of the remote ancestor. The placenta 

 the temporary organ, partly maternal, party foetal in 

 which the exchange of oxygen and nutrient substances 

 takes place between the maternal and the foetal circula- 

 tions, and which would have been not only useless but 

 injurious to a free-swimming animal, has been developed 

 since the ancestor of man carried its young for gradually 

 lengthening periods within its own body. 



As one would expect, the earlier stages are those where 

 the representation of the development of the race is most 

 inexact, for they have been longer subjected to selection 

 than the other periods. We must realise that different 

 characters have probably been subjected to different forms 

 of selection at different times, according to variations in the 

 environment. It is also very improbable that continuous 

 progressive variations can have occurred with regard to one 

 particular character or organ. Besides progressive varia- 

 tions occurring in one part of the body and regressive ones 

 in another at the same time, progressive and regressive 



