296 FIRST PRINCIPLES OF NATURAL CLASSIFICATION. 
But when a part of the series is perfect, and the other 
part presents the idea of a chain where several of the 
_ links are wanting, then the group 4s called imperfect. Now 
this imperfection arises from two causes: either these 
absent links have not yet been discovered, or they have 
been destroyed in the revolutions which have agitated 
our globe. ‘This is the first great law of the natural 
system ; it is that upon which all others repose, and which 
has been already demonstrated in almost every de- 
partment of zoology, but more especially in ornitho- 
logy. If the reader wishes to see this theory made 
good in the animal world, we must refer him to the 
Hore Entomologice and to the Northern Zoology. We 
may refer him, in the last-mentioned work, to the genus 
Picus, and to the sub-family Piciane, as examples of 
perfect groups ; and to the family Picide (of the same 
volume) for one that is imperfect. The circle of the 
animal kingdom (p. 203.) is also a familiar illustration 
upon a large scale. Commencing with the Polypes, we 
pass on to the Mollusca; from these we are led to verte- 
brated animals ; thence to insects and radiated animals ; 
and, finally, arrive once more among the polypes. Our 
course has thus been circular ; the two ends of the series 
meet ; and we have, theoretically, a natural group. 
(277.) II. We now pass to our second proposition ; 
viz. The primary circular divisions of every such group 
are three actually, and five apparently. 
(278.) Asit is manifest that every group, according to 
_ its magnitude, will exhibit more or less variety in its con- 
tents, the first question which suggests itself is, Are 
these variations regulated by any definite number? And 
is that number so constant, in all such groups as have 
been properly investigated, as to sanction the belief that 
itis universal P The answer isin the affirmative. Every 
group, whatever may be its rank or value, (that is, its 
size or its denomination,) contains, according to our 
theory, three other primary groups, whose affinities are 
also circular. One of these is called the typical, the other 
the sub-typical, and the third the aberrant group. This 
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