12 A JOURNEY IN BRAZIL. 
tribution of the carp family would afford many other 
striking examples, but they are too numerous and too little, 
known to be used as an illustration here. 
" This is among the most remarkable instances of what 
I would call the arbitrary character of geographical dis- 
tribution. Such facts cannot be explained by any theory 
of accidental dispersion, for the upper mountain rivulets, 
in which these great rivers take their rise, have no con- 
nection with each other ; nor can any local circumstance 
explain the presence of some species in all the three basins, 
while others appear only in one, or perhaps in two, and are 
absent from the third, or the fact that certain species 
inhabiting the head-waters of these streams are never found 
in their lower course when the descent would seem so 
natural and so easy. In the absence of any positive ex- 
planation, we are left to assume that the distribution of ani- 
mal life has primary laws as definite and precise as those 
which govern anything else in the system of the universe. 
" It is for the sake of investigations of this kind that 
I wish our party to divide, in order that we may cover as 
wide a ground as possible, and compare a greater number 
of the water-basins of Brazil. I wish the same to be d'one, 
as far as may be, for all the classes of Vertebrates, as 
well as for Mollusks, Articulates, and Radiates. As we 
have no special botanist in the party, we must be content 
to make a methodical collection of the most characteristic 
families of trees, such as the palms and tree ferns. A col- 
lection of the stems of these trees would be especially 
important as a guide to the identification of fossil woods. 
Much more is known of the geographical distribution of 
plants than of animals, however, and there is, therefore, 
less to be done that is new in that direction. 
