REVIEW OF FOSSIL FISHES. 245 



another, but that they appear and disappear 

 unexpectedly, without direct relations with 

 their precursors ; for I think no one will seri- 

 ously pretend that the numerous types of Cy- 

 cloids and Ctenoids, almost all of which are 

 contemporaneous with one another, have de- 

 scended from the Placoids and Ganoids. As 

 well might one affirm that the Mammalia, and 

 man with them, have descended directly from 

 fishes. All these species have a fixed epoch of 

 appearance and disappearance ; their existence 

 is even limited to an appointed time. And yet 

 they present, as a whole, numerous affinities 

 more or less close, a definite coordination in a 

 given system of organization which has inti- 

 mate relations with the mode of existence of 

 each type, and even of each species. An in- 

 visible thread unwinds itself throughout all 

 time, across this immense diversity, and pre- 

 sents to us as a definite result, a continual 

 progress in the development of which man is 

 the term, of which the four classes of verte- 

 brates are intermediate forms, and the totaUty 

 of invertebrate animals the constant accessory 

 accompaniment." 



The difficulty of carrying out comparisons 

 so rigorous and extensive as were needed in 

 order to reconstruct the organic relations be- 



