114 
SYSTEM OF NATURE. 
notice of any observant naturalist. It seems to me owing 
to the combined influence of three distinct principles: 1st, 
the existence of centres; 2ndly, the existence of parallels; 
3rdly, the existence of dichotomous external groups. Tak¬ 
ing the Crustacea as an example, I will state a case hypo¬ 
thetically by way of explanation. I will suppose it proved 
that the vertebrates and apterous articulates each contain 
four prominent groups, and that they correspond with each 
other, thus : — 
VERTEBRATA. APTERA. 
Placentalia. 
Aves. 
Reptilia. 
Pisces. 
Arachnida. 
Ametabola. 
Myriapoda. 
Crustacea. 
I will next assume that fishes are external and double in 
the vertebrates, and that Crustacea are central and single 
in the Aptera. In accordance with the theory of parallels, 
both groups of fishes must find representatives in the Crusta¬ 
cea : we can scarcely imagine this to be accomplished other¬ 
wise than in a sesquialterous manner, one portion vastly 
exceeding the other in importance: thus, the rapacious 
cartilaginous fishes, being the higher group as regards the 
class Vertebrata, would find a parallel in the rapacious 
Maxillosa or Malacostraca; and the abnormal and less ra¬ 
pacious bony fishes would find their parallel in the Eden¬ 
tata or Entomostraca. 
Returning from this short and I think necessary digres¬ 
sion on the sesquialterous character of certain groups — a 
phenomenon of almost constant occurrence,—I resume the 
consideration of the animal kingdom as expressed by a fi¬ 
gure of concentric circles. It will be observed that I have 
already stated my opinion of the groups circumscribed 
