ANIMAL PEDIGREES 241 



in the struggle for existence. It is not enough 

 that the ultimate stage should be more advantageous 

 than the initial or earlier condition, but each inter- 

 mediate stage must also be a distinct advance. If 

 then the development of an organ is strictly 

 recapitulatory, it should present to us a series of 

 stages, each of which is not merely functional, but 

 a distinct advance on the stage immediately pre- 

 ceding 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 pre- 

 sents difficulties 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 

 slightly modified patch of skin ; round the edge of 

 the patch the epidermis becomes raised up as a 

 rim ; this gradually grows inwards from all sides, 

 so that the depressed patch now forms a pit, 

 communicating with the exterior through a small 

 hole or mouth. By further growth the mouth of 

 the pit becomes still more narrowed, and ultimately 

 completely closed, so that the pit becomes converted 

 into a closed sac or vesicle ; at the point at which 

 final closure occurs formation of cuticle takes place, 

 which projects as a small transparent drop into the 

 cavity of the sac; by the formation of concentric 

 layers of cuticle this drop becomes enlarged into the 



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