PROGENITORS OF MAN, 287 



Eleventh Stage : Primaeval Fish (Selachii.) 



Of all known Vertebrate animals, the ancestors of the 

 Primseval Fish probably showed most resemblance to the 

 stiU living Sharks (Squalacei). They originated out of 

 the single-nostriled animals by the division of the single 

 nostril into two lateral halves, by the formation of a 

 sympathetic nervous system, a jaw skeleton, a swimming 

 bladder, and two pairs of legs (breast fins or fore-legs, and 

 ventral fins or hind-legs). The internal organisation of this 

 stage may probably, upon the whole, have corresponded to 

 the lowest species of Sharks known to us ; the swimming 

 bladder was however more strongly developed ; in the case 

 of the latter it exists only as a rudimentary organ. They 

 lived as early as the Silurian period, as is proved by the 

 fossil remains of sharks (teeth and fin spines) from the 

 Silurian strata. A certain proof that the Silurian ances- 

 tors of man and of all the other double-nostriled animals 

 were nearest akin to the Selachii, is furnished by the 

 comparative anatomy of the latter ; it shows that the 

 relations of organisation in aU Amphirrhina can be derived 

 from those of the Selachii. 



Twelfth Stage : Mud Fish (Dipneusta). 

 Our twelfth ancestral stage is formed by Vertebrate 

 animals which probably possessed a remote resemblance to 

 the still living Salamander fish (Ceratodus, Protopterus, 

 Lepidosiren, p. 212). They originated out of the Primseval 

 fish (probably at the beginning of the palaeolithic, or 

 primary period) by adaptation to life on land, and by the 

 transformation of the swimming bladder into an air- 

 breathing lung, and of the nasal cavity (which now opened 



