58 
SYSTEM OF NATURE. 
portance. That if, for instance, the group of Mammalia 
and that of birds be equal to each other, each of the other 
classes — that is to sag, every other group of the same 
rank — must be equal to each other ; and also, that the 
subordinate groups in each of these classes must exhibit 
the same mutual relations in every case. But if it can 
be shown that in one class, so called, two ordinal groups 
exhibit as great a discrepancy in their relative plan of 
organization as any two classes do, then the relation of 
the former to either of the latter is not, and cannot be the 
same, as that which exists between the latter two ; yet in 
this predicament stand the three first classes of the Ver- 
tebrata, the relations of the Mammalia and birds being 
much stronger and more obvious than those of the Beptilia 
to either, and the two groups of the latter, which I have 
just sketched — the tortoises and the serpents — being 
nearly or quite as far removed by their structure from 
each other as the birds are from the Mammalia ?* 
Having thus shown that a vast discrepancy exists at the 
supposed extreme points of the series, restricted as it is 
throughout these quoted observations to what are called 
true reptiles, it must next be enquired whether there exists, 
at any point of that series beginning with the tortoises 
and ending with snakes, any halting-place where one form 
decidedly ceases and another as decidedly begins. The 
question appears to me beset with difficulties, and these 
are not to be solved by an authoritative dictum : there is 
no sword wherewith to cut the Gordian knot at a single 
blow ; the entire series must be carefully considered, and 
its due importance given to every structural difference. 
* Bell’s British Reptiles, Int. viii. 
