198 THE EYES OF THE BLIND VERTEBRATES OF NORTH AMERICA. 



control the ontogeny as to make the latter into a record of the former — even into an 

 imperfect record of it." "Vestiges, and these only, can give any embryological clue 

 to past history which could not be equally well made out from comparative anatomy." 



Zittel finds cases in paleontology both in support of and against this first prop- 

 osition. 



"All know that it (development of Antedon) does not in the remotest manner 

 agree with the facts of paleontology." "No observations of embryology would 

 warrant our imagining the former existence of graptolites or stromatophores. No 

 stage in the development of any living brachiopod informs us that numerous 

 spine-bearing genera lived in Palaeozoic and Mesozoic times. . . . The beautiful 

 researches of Hyatt, Wiirtemberger, and Branco have shown that all ammonites 

 and ceratites pass through a goniatite stage, and that the inner whorls of an ammonite 

 constantly resemble, in form, ornament, and suture-line, the adult condition of some 

 previously existing genus or other." 



Smith (:00, p. 226) finds that "the development of Placenticeras shows that it 

 is possible to decipher the race history of an animal in its individual ontogeny." 



But it is not the intention to review the numerous expressions of opinion pro 

 and con which have appeared on this subject in recent years. A full discussion of 

 the literature to 1897 has been given by Keibel ('98). 



The eye of Amblyopsis presents, however, such an excellent opportunity to test 

 an opinion vaguely expressed by Balfour in his Embryology, and carefully and 

 clearly stated by Sedgwick ('94) and reiterated by Cunningham in his Sexual Dimor- 

 phism (:00) and in other places, that the facts presented in the foregoing pages may 

 be re-examined in their relation to this point. 



Balfour says ('85, vol. 2, p. 361) : "Abbreviations take place because direct devel- 

 opment is always simpler, and therefore more advantageous; and, owing to the fact 

 of the foetus not being required to lead an independent existence till birth, and of its 

 being in the mean time nourished by food-yolk, or directly by the parent, there are 

 no physiological causes to prevent the characters of any stage of the development 

 which are of functional importance during a free but not during a foetal existence 

 from disappearing from the developmental history. ... In spite of the liability of 

 larvse to acquire secondary characters, there is a powerful counterbalancing influence 

 tending towards the preservation of ancestral characters in that larvse are necessarily 

 compelled at all stages of their growth to retain in a functional state such systems of 

 organs, at any rate, as are essential for a free and independent existence. It thus 

 comes about that, in spite of the many causes tending to produce secondary changes 

 in larvse, there is always a better chance of larvae repeating, in an unabbreviated form, 



