30 A JOURNEY IN BRAZIL. 



Old World, and in all the rivers and lakes of the temperate 

 zone, communicating with the Atlantic Ocean. They occur 

 in smaller numbers in most tributaries of the Mediterra- 

 nean, but are common in the Volga and Danube, as well 

 as in the Mississippi, in some of the rivers on our north- 

 ern Atlantic and Pacific coasts, and in China. This fam- 

 ily has no representatives in Africa, Southern Asia, Austra- 

 lia, or South America, but there is a group corresponding 

 in a certain way to it in South America, that of the Go- 

 niodonts. Though some ichthyologists place them widely 

 apart in their classifications, there is, on the whole, a 

 striking resemblance between the Sturgeons and Gonio- 

 donts. Groups of this kind, reproducing certain features 

 common to both, but differing by special structural modifica- 

 tions, are called ' representative types.' This name applies 

 more especially to such groups when they are distributed 

 over different parts of the world. To naturalists the com- 

 parison of one of these types with another is very interest- 

 ing, as touching upon the question of origin of species. To 

 those who believe that animals are derived from one another 

 the alternative here presented is very clear : either one of 

 these groups grew out of the other, or else they both had 

 common ancestors which were neither Sturgeons nor Goni- 

 odonts, but combined the features of both and gave birtty to 

 each. 



" There is a third family of fishes, the Hornpouts or Bull- 

 heads, called Siluroids by naturalists, which seem by their 

 structural character to occupy an intermediate position be- 

 tween the Sturgeons and Goniodonts. There would seem 

 to be, then, in these three groups, so similar in certain fea- 

 tures, so distinct in others, the elements of a series. But 

 while their structural relations suggest a common origin, 



