939 STUDY OF NATURAL HISTORY. 
the animal kingdom, and examine the question 
more Closely. 
(161.) The thick-skinned or pachydermatous tribe 
of quadrupeds comprise the genera of the elephant, 
rhinoceros, megatherium, and hippopotamus: these 
are well known as the most gigantic of all animals. 
We have ascertained, by analysis, that they form a 
circular group, and that the rank of this group: is 
equivalent to that of a tribe. Yet, in regard to the 
number of objects it comprises, this is the most 
scanty tribe in the animal kingdom. It does not 
contain, in fact, as many individuals as are found 
in a single genus of parrots. Whence, therefore, 
arises this disparity? How are we to account for 
the wide intervals between the different Pachyder- 
mata, and the very small ones between the genera 
of parrots? To this we should answer, first, that 
many of these forms, which once existed, are lost ; 
and, secondly, that their paucity, so far from dis- 
turbing the harmony and regularity of nature’s 
system, tends to show it in a light directly the re- 
verse. First, then, the extinction of numerous forms 
of Pachydermata rests on well known and incon- 
trovertible facts. Not only are the fossil remains 
of hippopotami, of elephants, and of rhinoceroses, 
belonging to extinct species (and very probably to 
intermediate gradations of form), found in nume- 
rous and various parts of the world, and in consi- 
derable quantities, but modern geology has brought 
to light a whole family of these quadrupeds, repre- 
sented by the megatherium, which are now so com- 
pletely exterminated from the earth, that not a single 
living example exists to testify the creation of such 
