206 THE HISTOEY OF CREATION. 



sac, and a spleen. Further, all Double-nostriled animals 

 possess a bladder-shaped expansion of the gullet, which, in 

 Fish, has developed into the swimming bladder, but in all 

 other Double-nostriled animals into lungs. Finally, in all 

 Double-nostriled animals there exist in the youngest stage 

 of growth the beginnings of two pairs of extremities, or 

 limbs, a pair of fore legs, or breast fins, and a pair of hinder 

 legs, or ventral fins. One of these pairs of legs sometimes 

 degenerates (as in the case of eels, whales, etc.), or both 

 pairs of legs (as in Csecilise and serpents) either degenerate 

 or entirely disappear ; but even in these cases there exists 

 some trace of their original beginning in an early embryonic 

 period, or the useless remains of them may be found in the 

 form of rudimentary organs. (Compare above, vol. i. p. 13.) 



From all these important indications we may conclude 

 with full assurance that all double-nostriled animals are 

 derived from a single common primary form, which 

 developed either directly or indirectly during the primordial 

 period out of the Monorrhina. This primary form must 

 have possessed the organs above mentioned, and also the 

 beginning of a swimming bladder and of two pairs of legs 

 or fins. It is evident, that of all still living double-nostriled 

 animals, the lowest forms of sharks are most closely allied 

 to this long since extinct, unknown, and hypothetical 

 pi'imary form, which we may call the Primary Double- 

 nostriled animals (Proselachii). We may therefore look 

 upon the group of primseval fish, or Selachii, to which the 

 Proselachii probably belonged, as a primary group, not 

 only of the Fish class, but of the whole main-class of double- 

 nostriled animals. 



The class of Fish (Pisces) with which we accordingly 



