212 STUDY OF NATURAL HISTORY. 
is arrested. He has set out with assuming as 
correct, one great law of nature,—the circular 
progression of affinities: but now he is to en- 
quire, before he can proceed further, on what prin- 
ciples he is to connect his sub-genera, so as to 
preserve their affinities, and yet form them into 
assemblages of a higher order or superior value. 
The answer, theoretically, is obvious to every one 
accustomed to logical reasoning. If, in all natural 
groups, the progression of affinity is circular, then 
the contents of a genus, which is a natural group, 
must be circular also. Such is the application of 
the law in question; for it cannot be supposed that 
the higher divisions of nature, as classes and orders, 
should demonstrate this circularity, and that the other 
subordinate groups should not: this, were it true, 
would disprove a unity and consistency of plan, and 
the law, not being general, would be no law. A 
genus, then, to be natural, must only contain, of 
necessity, such sub-genera or minor assemblages of 
species as will, collectively, show a circular progres- 
sion of affinity. Such is one of the requisites of a 
natural genus. Still the question of numbers is to 
be investigated. What are the natural divisions of 
a group? in other words, how many of those, here 
called sub-genera, constitute a genus ? 
(148.) Now, as we are proceeding analytically, 
under no assumed law but that which regards the 
circular theory of affinity, we must have recourse to 
observation. If we find that three, or four, or 
any other definite number of sub-genera, by being 
placed together, appear to form a circular group, 
Ce as 
