MEANING OF ORDERS. 73 



ne 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 

 lind 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- 



