ON" THE ORIGIN OF GENERA. 113 



We may also compare such extended metamorphoses with 

 those of cosmical matter, such as when, in the course of ages, a 

 primeval vapor has in a short time collapsed to the liquid form, or 

 as when the vast of liquid in turn has shrunk to its solid con- 

 dition ; both alike for ages approaching their change, yet sta- 

 tionary in external relations till the moment of transition has 

 arrived. 



The following are the zoological relations of the groups al- 

 ready referred to : 



The most generalized group of fishes of the Regio Neotropica 

 is that of Characins. Its type, in respect to fin-structure, which 

 is common to all the Malacopterygians, is that of an undeveloped 

 stage of the Acanthopterygians, the adipose fin being an undevel- 

 oped cartilaginous fin, and the cartilaginous fin an undeveloped 

 spinous fin.* It may be said to be the highest among Malacop- 

 terygians, if we look to the conq^lete oviducts, opercula, jaws, etc.,t 

 but it is the lowest as removed farthest from the extreme of 

 Malacopterygian peculiarities, as being most generalized or em- 

 bracing representatives of all the rest, and approaching nearest 

 the types of the past — the Ganoids. Tor example, Butyrinus and 

 Vastres may be compared with Amia. The family is distributed 

 chiefly in the Southern Hemisphere. 



The genus Orestias, which Agassiz says is characterized by a 

 feature which exists in the immature state of all other Oyprino- 

 donts — the absence of ventral fins — is only found in the Neotrop- 

 ical region. 



Of the yenomous serpents, the inferior group, the Proterogly- 

 pha, belong to the Southern Hemisphere, and the Australian and 

 Neotropical regions almost exclusively embrace by far the greater 

 proportion. Australia contains none other. 



The Iguanian lizards are lower than the Acrodont, exhibiting 

 a larval type of detention, and one characteristic of all lower Sau- 

 ria and Batrachia. The only acrodont type of Ophiosaura (Trog- 

 onophis) is Old World. 



The New World Teidas have not the extent of ossified tem- 

 poral roof that their representatives, the Old World Lacertidae, 

 have. So the chiefly Neotropical Anguidae haye the tongue part- 



* Kner, " Ueber den Bau der Flosscn." 



\ This is the correct view, for this family and the Siluroids are the most special- 

 ized of the Malacopterygian fishes. (1886.) 

 8 



