GROUPS NOT ALWAYS PERFECT. 213 
We may; in this stage of our enquiry, have some 
reason to suppose that it is a natural one, because it 
exemplifies the law in question. 
(149.) It has been found, however, that this is 
not a sufficient verification of a natural series; and 
for this reason: in the infinite variety of animal forms, 
there are so many mutual resemblances, that if our 
only object is to arrange them in circles, we may 
combine them in different ways, all of which will 
wear a primd facie appearance of being more or 
less circular ; or if any unusual Azatus or gaps appear, 
we are immediately ready to smooth over the diffi- 
culty, by concluding that they do not really exist in 
nature, but only in the paucity of our materials. 
And we are the more inclined to yield to this per- 
suasion, since naturalists universally admit that such 
intervals really do exist in nature, either from the 
extermination of some animals by man, or from the 
changes which our earth has undergone. Against 
this disposition, therefore, to smooth over discrepan- 
cies in our supposed circle, by attributing them to 
causes which may or may not be the true ones, 
the naturalist must frequently contend. He may 
rest assured that a natural group can only have 
one series of variation; and if, by taking away 
some of its parts, and substituting others, he 
can still preserve its circular appearance, he has 
strong reason to doubt which disposition is the 
natural one. 
(150.) Now, under these circumstances, the ef- 
fective mode of proceeding is to form two or more 
of those combinations called genera, and then to com- 
pare their contents respectively with each and all. 
Pg 
