30 A JOURNEY IN BRAZIL. 



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

 zone, communicating with the Atlantic Ocean. Thev occur 



y ^j / 



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, cr 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 birth 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. 



