296 STUDY OF NATURAL HISTORY. 
more particularly of perfect groups; that is, of such 
as exhibit, in their circular progression, no wide or 
disproportionate gaps in their continuity. The na- 
turalist, however, must not calculate on frequently 
falling upon a cluster of these, so near to each other, 
that every genus, for example, ina sub-family, shall 
be perfect. How, then, is he to proceed, since he 
cannot, in all instances, verify the law he has set 
out with assuming, 7.e. that every natural group is 
circular ? He must, in this dilemma, in the first in- 
stance, chiefly be guided by observation. Should he 
find that, by bringing together a certain number of 
groups, they will form a circle more or less com- 
plete, and of a higher denomination, he may, in the 
first instance, assume that the law in question has 
been complied with, if not in the letter, at least in 
the spirit. Some of the groups, thus united into one, 
may be perfect ; whereas others may contain very few 
objects, and these objects, having distinct intervals 
between them, form imperfect groups; that is, they 
present such distinct and unequal spaces in the line 
of continuity, as to impress us with a conviction that 
intermediate forms are wanting, to render the circle 
perfect. Nay, it will sometimes happen that these 
last-mentioned groups contain but two or three in- 
dividuals, while the others comprise forty or fifty. 
In cases like these, we must endeavour to discover 
how far these two or three individual forms, — 
placed together as the fragments, so to speak, of a 
circle,—are represented in the more perfect of the 
adjoining groups ; and by the degree of continuity 
which these latter exhibit, estimate the extent of the 
hiati. If these isolated forms are represented in 
a? 
