PROGENITORS OF MAN. 287 
ELEVENTH Stace: Primeval Fish (Selachii.) 
Of all known Vertebrate animals, the ancestors of the 
Primeval Fish probably showed most resemblance to the 
still 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 asa 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 all 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 Primeval 
fish (probably at the beginning of the palzolithic, 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 
