MEANING OF ORDERS. 73 
he will establish the rank of the order upon that 
feature, while some other naturalist, appreciating 
a different point of the structure more highly, 
will make that the test character of the group. 
Let us see whether we can eliminate this arbitra- 
ry element in our estimate of these groups, and 
find any mode of determining orders that shall 
be unquestionable, and give us results as positive 
as a chemical analysis according to quantitative 
elements. I believe that there are such absolute 
tests of structural relations. It is my conviction, 
that orders, like all the other groups of the 
Animal Kingdom, have a positive existence in 
Nature with definite limits; that no arbitrary. 
element should enter into any part of our classi- 
fications; and that we have already the key by 
which to solve this question about orders. 
To illustrate this statement, I must return to 
the class of Insects. We have seen that they 
are divided into three orders: the long cylindri- 
cal Centipedes, with the body divided throughout 
in uniform rings, like the worms; the Spiders, 
with the body divided into two regions; and the 
Winged Insects, with head, chest, and hind body 
distinct from each other, forming three separate 
regions. In the first group, the Centipedes, the 
nervous system is scattered through the whole 
body, as in the Worms; in the Spiders it is con- 
centrated in two nervous swellings, as in Crusta- 
4 
