DEVELOPMENT OF THE EYE OF AMBLYOPSIS. 171 



former — even into an imperfect record of it. * * * Vestiges, and these only, can give any em- 

 bryological clew 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 propo- 

 sition : 



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 PaLxozoic 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 re- 

 semble, in form, ornament, and suture-line, the adult condition of some previously existing genus 

 or other. 



Smith 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. 



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 and reiterated by Cunningham in his " Sexual Dimor- 

 phism " and in other places, that the facts presented in the foregoing pages may 

 be reexamined in their relation to this point. 



Balfour says: 



Abbreviations take place because direct development is always simpler, and therefore more 

 advantageous; and, owing to the fact of the foetus not being required to lead an independent exist- 

 ence 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 fatal, existence from disappearing from 

 the developmental history. * * * In spite of the liability of larva; to acquire secondary characters, 

 there is a powerful counterbalancing influence tending toward the preservation of ancestral char- 

 acters in that larvae are necessarily compelled at all stages of their growth to retain in afunctional 

 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 larvae, 

 there is always a better chance of larva? repeating, in an unabbreviated form, their ancestral history 

 than is the case with embryos which undergo their development within the egg. 



The most concrete critique of the law of biogenesis has been offered by Sedgwick. 

 After rejecting the second proposition by showing that, while in many cases the 

 adults differ more from each other than the young, in other cases the embryos differ 

 more from each other than the adults, he takes up the main question stated in the 

 first proposition by a consideration of "The Significance of Ancestral Rudiments in 

 Embryonic Development." It is, indeed, around this phase of the subject that 

 the discussion has centered. His views are best given by a series of excerpts from 

 his paper. Thus Sedgwick states that 



* * * The tendency in embryonic development is to directness and abbreviation and to the 

 omission of ancestral stages of structure, and that variations do not merely affect the not-early 

 period of life where they are of immediate functional importance to the animal, but, on the contrary, 

 that they are inherent in the germ and affect more or less profoundly the whole development. 



The evidence is of this kind : 1. Organs which we know have only recently disappeared are not 

 developed at all in the embryo. For instance, the teeth of birds, the fore limbs of snakes, reduced 



