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J, D. Dana on Megasthenes and Microsthenes. 9 
characteristic may oe be detected corresponding to these m 
thenic and microsthenic qualities; but even then, the distinctive 
idea of the subdivisions er hardly be better expressed than 
by the names proposed. 
The parallelism between the Megasthenes and Microsthenes 
among Mammals and the Decapods and pig spay among 
Crustaceans suggests, that if the subdivisions be called orders 
in the latter case, they should be so called in the Semmes 
The distinction betwee egasthenes and Microsthenes may, 
rhaps, become more retathigttie if we regard a living structure 
as a life-system, or, speaking dynamically, a life-battery. In order 
that such batteries may have a very wide range of size, two or 
more plans of construction, more or Jess different, appear to be 
requisite. With one plan, there is a certain magnitude which 
is that of most efficient action and power; and from this magni- 
tude, there may be a series of larger and ‘smaller _ reaching 
to the outer limits of norma perfection ; and then, if these 
limits be passed in either direction, that is, either on he side of 
too great magnitude, or of oe little, degradation ; in the structure 
and its powers begin to a ; 
To carry the species ghrbah another range of sizes, with nor- 
perfection of structure, another somewhat differ ent plan is 
required. The Megasthenes represent one such plan, the Mic- 
rosthenes another. 
is idea is brought out by the writer in his chapter on the 
Classification of Crustaceans already referred to. He there says, 
eaking of the orders of et: viz: Decapods, Tetra- 
ecapods, —< Entomostracan 
other words, the sy ert is of different orders for the different types, 
and the structures formed exhibit ~~ extent of their spheres of action, 
such as are gaan ny use the force most effectively, in accordance 
given type, » as the first, for example, the same “Ah ones may 
be of different « bobs webs ada\ to structures of different sizes. But 
the size in either direction for structures of efficient ‘ction is limited, 
To pass these limits, a life-system of another order is required. The 
Macroura, as they diminish in size, finally pass this limit, and the organ- 
isms (Mysidz, for example) are no longer perfect in their members; an 
obsolescence of some p: ns to take place, and species of this small 
Tetra are actually complete 3 when provided with the structure 
etradecapod. 
The extreme size of structure admitting of the highest efficient ac- 
vty Speed three to six times lineally the average or mean typical 
these gigantic species, three or four times longer than the 
a Jour. — onD Series, Vou. XXXVI, No. 106.—Juny, 1863. 
