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 



O ' 



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 totality 

 of invertebrate animals the constant accessory 

 accompaniment." 



The difficulty of carrying out comparisons 

 so rigorous and extensive as were needed in 



o 



order to reconstruct the organic relations be- 



