56 The Recapitulation Theory and Human Infancy 



acter which was carried down through embryogeny in descent. 

 This, it will be recalled, Morgan did not attempt to explain. 

 This hypothesis of Sedgwick 92 was accepted and applied by 

 two other biologists. Cunningham carried the discussion back 

 to the probable ancestor of the Amniota to which the larval 

 condition may be assigned. It is not likely, he says, that the 

 case of Salamander atra is illustrative of the absorption of the 

 branchiate condition among mammals, 



"Since viviparity even in Mammals was evidently acquired 

 at a later stage, and the change came about by means of the 

 development of an egg-shell in which the larva was contained 

 and protected during its development." Moreover, "the mere 

 fact that they were functional originally in a free larval condi- 

 tion is probably not the whole explanation of the temporary 

 development of a complete system of gill-arches and gill-slits 

 in the amnoite embryo. Supposing that no functional import- 

 ance in embryonic life belongs to them, it is to be noted that the 

 embryonic branchial blood vessels are metamorphosed into the 

 vascular system of the adult, as they are in the metamorphosis 

 of the Amphibia, and a very considerable and very important 

 part of them persists in the adult structure. This does not ac- 

 count for the persistence in the embryo of open branchial clefts, 

 but even these do not wholly disappear in development. The 

 first remains as the Eustachian tube, and even the posterior 

 are related to the thymus gland. It seems to be a general fact 

 that a structure which in metamorphosis disappears completely 

 may easily be omitted altogether in embryonic development, 

 while one which is modified into something else continues to 

 pass more or less through its original larval condition. If the 

 aquatic larval condition then was ever an adult ancestral con- 

 dition it must have been the ancestor of Amphibian forms, and 

 only through these the ancestor of the Amniota." 93 



The adapted character of these larval forms which reflect in 

 some degree an ancient adult condition is made clear by Cunning- 

 ham, and is of special significance for this whole class of recapitu- 

 latory cases. From a comparison of the tadpole and the known 

 forms of fishes he concludes that 



"The larva of the frog, therefore, instead of recapitulating 

 the ancestral condition has lost nearly everything characteristic 

 of a fish except the pharyngeal branchiae, which it could not 



•> Cunningham notes that the same suggestion was made antecedently by Bal- 

 four. See Comparative Embryology, 2d ed., 1885, p. 210. 

 " Science Progress, VI, 1897, p. 488. 



