4 
SYSTEM OF NATURE. 113 
Although I am unable to adduce instances from the pre¬ 
ceding pages in support of such an opinion, I will yet ven¬ 
ture to record my firm belief that pairs similar to these 
occur in all minor divisions. We even find pairs of spe¬ 
cies : it is a circumstance of frequent occurrence to meet 
with two species so extremely similar that it requires the 
instructed eye of science to detect a difference between 
them. 
I now revert to the fourth group, which I have described 
(p. Ill etpassim) as ‘central and single.’ Although this 
definition may be correct when contrasting it with the ob¬ 
viously double groups by which it is surrounded, yet when 
submitted to an isolated analysis, this central group will 
be found again divisable into two portions, one much more 
extensive and more important than the other. This kind 
of dichotomy may be represented by what has been called 
a sesquialterous circle. As examples of these sesquialter- 
ous centres, I may cite the Crustacea and Neuroptera : the 
former comprising the normal Maxillosa and abnormal 
Edentata, and the latter the normal net-winged Neuroptera 
and abnormal Phryganese; the abnormal groups being so 
decidedly discrepant, that naturalists have not hesitated 
to raise them to a rank equal to that of the normal groups, 
the names Entomostraca and Trichoptera having been ap¬ 
plied to them. These instances of sesquialterous centres 
are not peculiarly striking, but are the only ones occurring 
in the five groups which have been considered in the pre¬ 
ceding pages; the other centres, Primates, Placentalia and 
Vertebrata, appear single, a peculiarity which I suppose at¬ 
tributable to their centrality as regards the animal kingdom. 
This disposition of groups to contain two most unequal 
yet well-marked divisions, can scarcely have escaped the 
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