RANK OF CIRCULAR GROUPS. 549 
cularity can be traced ; but in some few instances this 
can be done: we are, therefore, justified in believing 
they would invariably possess this property, were they, 
in all instances, equally abundant in species. Sub- 
genera thus become the lowest circular groups in nature, 
or, in other words, they are the first assembling together 
of species: all of which belong to the same type of form- 
ation. Nearly all the modern genera are, therefore, in 
fact, sub-genera, because every deviation from a type of 
formation is made into “a genus ;” but no effort is made 
to assemble these types under those intervening groups, 
which come between genera and families. It has never- 
theless been repeatedly -proved to demonstration that two 
intervening circular groups do actually exist in nature ; 
and which, as already stated, are bond fide genera and 
sub-families. The modern practice of defining and naming 
these sub-genera is very useful, provided their true dis- 
tinctions are conspicuously noted ; but, as the theory of 
definite variation (415.) has hitherto received no atten- 
tion, at least in entomology, it frequently happens that 
the essential characters are overlooked, and the unimport - 
ant ones brought forward. As an example of sub-genera 
really natural, we must refer to the “‘ Zoological Ilus- 
trations,’ where the reader will find all those of the 
genus Po/yommatus described and figured. It will be fur- 
ther remarked, that these sub-genera are to be regulated 
in their selection by the same laws as are applicable 
to higher groups. Their individual circularity, indeed, 
cannot be always traced, unless they are united in their 
proper genus, as in Polyommatus ; they will then re- 
present all the higher divisions of their own order. The 
sub-genus Polyommatus, for instance, stands at the head, 
as representing Papilio Lath. Next follows Lycena, 
or the copper butterflies, united most completely to the 
last, and representing Mymphalis Lath. Nats comes 
next, as typical of the Hesperide. Lucia represents the 
Erycinide ; and Erina brings us again to Polyommatus, 
with which it not only unites, but represents in itself 
the Satyride. Now, each of these, according to the 
