168 
TODUS. 
he will perceive that the entire genus Todus is too 
distinct and peculiar to he incorporated with either 
Rhipidura, Monacha, or Megalophus ; while its re- 
lation to this latter, and to the genus Muscicapa, is 
so very nearly equal, that we can only conclude its 
station to be intermediate. This point being settled, 
we are next to ascertain whether Todus , — as a 
whole, — constitutes a natural group ; or, in other 
words, whether this group is circular and represen- 
tative. This question, as will presently appear, can 
also be answered in the affirmative. Hence it fol- 
low's, that if its sub-genera differ more among them- 
selves than do the species which we have been 
illustrating, our inevitable conclusion is, that these 
latter are variations of the sub-genus, and not of the 
genus , — that they are specific modifications, and not, 
like Conopophaga, &c., types of sub-genera. We 
shall arrive at a similar conclusion if we argue the 
question in another w r ay. We have stated, in ano- 
ther place, as one of the primary laws of natural 
classification, that the number of circular groups, in 
point of rank, are Nine; and that those which 
form the ninth are sub-genera. Let us then take 
one of the examples that have been quoted in sup- 
port of this theory, and see how far the group, 
which w r e here term a sub-genus, will agree there- 
with. We have stated that Parus biarmicus holds 
precisely the same rank in the typical division, or 
sub-genus of Parus, as does Todus platgcircus in 
our sub-genus Todus ; the two birds, in fact, repre- 
senting each other. We will therefore now see how 
