40 The Recapitulation Theory and Human Infancy 



but upon the tendency of the mind to pass from the apparently 

 simple to the manifestly complex, and to regard the former as 

 primitive and ancestral and the latter as secondary and deriva- 

 tive."" 



Another class of facts representing this third type of evidence 

 for recapitulation is made up of certain profound metamor- 

 phoses observable in the development of certain groups possess- 

 ing larvae which recall in a rather striking way the ancient 

 forms from which groups in question are believed to be descend- 

 ed. These facts bear unmistakable evidence to a course of 

 evolution in parts of some lines of descent, in which variations 

 or mutations leading to the higher form have supervened em- 

 phatically at the adult end of the ontogeny. Ordinarily these 

 larvae have been adapted in important ways to the special 

 conditions of their life as larvae, but not so seriously as to wholly 

 obscure the ancient structures upon which the adaptations 

 took place. These larval forms are untrue to the ancient 

 pattern also by virtue of the fact that they are transient, con- 

 stantly progressing in the direction of the adult condition, 

 and their organization is therefore disposed in part with refer- 

 ence to the demands of the future. The customary example 

 is the familiar metamorphosis of the common frog. Similar 

 cases are found among the ascidians, the crustaceans, the in- 

 sects, the flat fishes. The only way in which the ancestral 

 reference in these cases is denied is by so minimizing the resem- 

 blance to the ancient form that the recapitulation is said to be 

 wholly superficial and lacking in real significance. This has 

 been done by a few extreme critics of the recapitulation theory. 

 Currect opinion of this class of facts is reported below. 



Finally these are those rudiments and isolated special char- 

 acters in the embryos of higher vertebrates which recall sim- 

 ilar characters and conditions in the adults of lower vertebrate- 

 forms. These are referred to by Sedgwick as follows: 



"... .the embryos of the higher vertebrate possess in the struc- 

 ture of the pharynx and of the heart and vascular system cer- 

 tain features — namely, paired pharyngeal apertures, a simple 

 tubular heart, and a single ventral aorta giving off right and left 



" Smith, Primitive Animals, pp. 14 el seg. 



For an elaborate hypothesis regarding the evolution of animal forms largely on 

 the type of evidence now under discussion, see Bernard, Some Neglected Factors in* 

 Evolution, 1911. 



