346 THE RECAPITULATION THEORY 



stages must be possible ones, the history must be 

 one thatTOuld actually have occurred i.e., the 

 several steps of the history as reconstructed must 

 form zTseries, all the stages of which are practicable 

 ones.^_Natural selection explains the actual structure 

 of a ^complex organ as having been acquired by the 

 preservation of a series of stages, each a distinct 

 if ^slight advance on the stage immediately pre- 

 ceding it, an advance so distinct as to confer on its 

 possessor an appreciable advantage in the struggle 

 forjExistence. It is not enough that the ultimate 

 stage should be more advantageous than the initial 

 oF earlier condition, but each intermediate stage 

 must also be a distinct advance. If then the 

 development of an organ is strictly recapitulatory, 

 ft shouldjresent to us a series of stages, each of 

 which is not merely functional, but a distinct 

 advance on the stage immediately preceding it. 

 Intermediate stages e.g., the solid oesophagus of 

 the tadpole which are not and could not be 

 functional, can form no part of an ancestral series ; 

 a consideration well expressed by Sedgwick* thus : 

 " Any phylogenetic hypothesis which presents diffi- 

 culties from a physiological standpoint must be 

 regarded as very provisional indeed." 



A good example of an embryological series 

 fulfilling these conditions is afforded by the de- 

 velopment of the eye in the higher Cephalopoda. 

 The earliest stage consists in the depression of a 



* Sedgwick, " On the Early Development of the Anterior Part 

 of the Wolffian Duct and Body in the Chick," Quarterly Journal 

 of Microscopical Science, vol. xxi. 1881, p. 456. 



