54 The Recapitulation Theory and Human Infancy 



have been handed down to the divergent lines of its descend- 

 ants. " 90 



In the opinion of Sedgwick the facts implying an ancestral 

 reference have been much exaggerated, but a sufficient number 

 of unequivocal cases do exist to require explanation. His ex- 

 planation follows: 



. "A disappearing adult organ is not retained in a relatively 

 greater development by an organism in the earlier stages of its 

 individual growth unless it is of functional importance to the 

 young form. In the cases in which the whole development is 

 embryonic this rarely happens, because the conditions of em- 

 bryonic life are so different from free life that functional embryon- 

 ic organs are usually organs sui generis, e. g., the placenta, am- 

 nion, etc., which cannot be traced to a modification of organs 

 previously present in the adult. It does, however, appear to 

 have happened sometimes, as an instance of it may be mentioned 

 the ductus arteriosus of the Sauropsidan and Mammalian 

 embryo. On the other hand, when there is a considerable period 

 of larval life, it does appear that there is a strong case for thinking 

 that organs which have been lost by the adult may be retained 

 and made use of by the larva. The best known example that 

 can be given of this is the tadpole of the frog. Here we find 

 organs, viz., gills and gill-slits, which are universally regarded as 

 having been attributes of all terrestrial Vertebrata in an earlier 

 and aquatic condition, and we also notice that their retention 

 is due to their being useful on account of the supposed ancient 

 conditions of life having been retained. Many other instances, 

 more or less plausible, of a like retention of ancestral features 

 of larvae might be mentioned, and it must be conceded that 

 there are strong reasons for supposing that larvae often retain 

 traces, more or less complete, of ancestral stages of structure. 

 But this admission does not carry with it any obligation to ac- 

 cept the widely prevalent view that larval history can in any 

 way be regarded as a recapitulation of ancestral history. Far 

 from it, for larvae in retaining some ancestral features are in no 

 way different from adults ; they differ only from adults in the feat- 

 ures which they have retained. Both larvae and adults retain an- 

 cestral features, and both have been modified by an adaptation 

 to their respective conditions of life which has ever been becom- 

 ing more perfect. 



The conclusion, then, has been reached, that whereas larvae 

 frequently retain traces of ancestral stages of adult structure, 

 embryos will rarely do so; and we are confronted again with 

 the question, How can we account for the presence in the embryo 



Evolution and Adaptation, pp. 73, 83. 



