218 STUDY OF NATURAL HISTORY. 
cidenée, we must yet bear in mind that we have 
advanced but one step in the scale of induction ; 
that it is not very difficult, even with a strict atten- 
tion to the foregoing rules, to divide two genera, 
each into the same number of sections. To assume, 
‘on such slender premises, the existence of a general 
law, that all genera will be found similarly constituted, 
would be a total departure from that mode of enquiry 
which is absolutely essential to the prosecution of 
all physical science. 
(154.) When, therefore, we have sabe the 
prevalence of a definite number in the divisions of 
a genus, by comparing the contents of several, we 
may then advance a step further, and are at liberty, 
from the facts already elicited, to form a theory. 
It is essential, however, that, in so doing, we over- 
step not those inductions which lie before us, and 
which can be appealed to as instances of particular 
verification, and as presumptive evidences of the 
universality of the law assumed. By the process of 
investigation we are now pursuing, all deviations 
from the law we assume, must be accounted for on 
sound principles, or by the probable operations of 
known effects. Thus, for instance, many groups 
which, from having been already analysed and de- 
monstrated, we know to be genera, contain but two 
or three divisions, others four, and many but one. 
Now, as this occasional paucity of forms in a genus 
may be accounted for by various natural causes 
(148.), we are not hastily-to conclude that there is no 
definite number in nature, or that a genus may 
contain from one to twenty sub-genera, for aught 
we know to the contrary. Such imperfect groups, — 

