The Recapitulation Theory in Biology 53 



summarized. Passing in review the most prominent of the 

 cases that have been offered in support of recapitulation, namely, 

 the gill-clefts of reptiles, birds, and mammals, the notocord in 

 the vertebrate series, the development of the skull, of the kid- 

 neys, of the heart, with some others, and noting in some de- 

 tail in each case the likeness of the ontogenetic formation to 

 the corresponding conditions and functions at different levels 

 in the chordate series, he is impressed by the following facts. 

 Certain differentiations, such as external gills in the tadpole, 

 do not seem to have any representation among adults in the 

 lower groups. Such transformations as those of the tadpole, 

 can only be regarded as at best analogous to ancestral transi- 

 tions. More especially, the gill-clefts appear as early in the 

 development of the mammal as in the salamander or fish, which 

 seems paradoxical if the appearance in the mammal is a repeti- 

 tion of the adult amphibian stage. The case is not greatly 

 different with the notocord, skull, kidneys, and heart. Again, 

 in the cases of rudimentary teeth in birds, whales, and other 

 animals, it is manifestly absurd to claim that the ancestral adult 

 condition is repeated when rudiments only appear. Finally, 

 the possible complete loss of ancestral stages is shown in the 

 lack of any indication in the ontogeny of the snake of the limbs 

 of its lizard-like ancestors. Cases of the repetition of adult 

 ancestral stages outside the group of vertebrates, he avers, are 

 often doubtful, sometimes little less than fanciful. Moreover, 

 the work of certain students among the invertebrates shows that 

 common embryonic forms are found which could not possibly 

 be regarded as due to the retention of adult ancestral forms. 



How, then, are the apparent resemblances between the em- 

 bryos of higher forms and the adult of lower animals to be ex- 

 plained? 



"The answer is that this resemblance is deceptive, and in so 

 far as there is a resemblance it depends on the resemblance of 

 the adult of the lower form to its own embryonic stages with 

 which we can really make a comparison. The gill-slits of the 

 «mbryo of the chick are to be compared, not with those of the 

 adult fish, but with those of the embryo of the fish. . . .It is 

 only the embryonic stages of the two groups that we are justified 

 in comparing; and their resemblances are explained on the as- 

 sumption that there has been an ancestral adult form having 

 these embryonic stages in its development and these stages 



