256 THE CANADIAN NATURALIST. [^Ug. 



this consideration of rank. Of tliese humbler members of our 

 classes we may mention ih.Q Marsupials and the 3Ionotremes among 

 the mammals, the Ampliibia among the reptiles, the Mites among 

 the arachnidans, the Myrlapods among the insects, the Entozoa 

 among the worms. Indeed it is quite possible on this ground to 

 divide each of our clai^ses into two or more Suh-classes. This is 

 sometimes convenient for the sake of more accurate definition ; 

 but it is not necessary, since the division into orders sufficiently 

 expresses these grades of complexity or elevation. 



7. Division of Classes into Orders and Families. 



Orders, as already stated, are based principally on rank or 

 grade, to be ascertained by relative complexity or by the develop- 

 ment of the higher nature of the animal. The last section, how- 

 ever, obliges us to take this with some limitation ; for since we 

 have four descriptions or sorts of classes, each of these must have 

 the grade within it ascertained on special grounds. For example, 

 the orders of birds, insects, gasteropods, and acalephse, should 

 be ascertained chiefly by reference to the locomotive organs, as 

 being the system of organs most eminently represented in the 

 class. If we glance for a moment at the systems which have 

 been proposed, we shall see that this view has unconsciously com- 

 mended itself to naturalists. The orders of insects, for exam- 

 ple, are very plainly based on such characters, being founded 

 mainly on the wings. This is nearly equally manifest in the 

 ordinarily received orders of birds. It appears in the division 

 into Pteropods, Heteropods, and Gasteropods proper among the 

 Gasteropoda. It is also seen in the orders Ctenophora, Disco- 

 pliora^ Siplionophora, among Acalephse. It would be easy to show 

 by a detailed review of the orders in the animal kingdom, that, 

 in so far as they have been distinctly defined, they have in most 

 cases been framed with a reference to the prevailing characteristics 

 of the class ; and also with the idea of grade or rank as a leading 

 ground of arrangement. As previously observed, also, it is in the 

 construction of orders, and in ascertaining rank in other divi- 

 sions, that embryology and the doctrine of cephalisation are 

 chiefly useful. For the present, however, we must leave this sub- 

 ject until we shall have an opportunity to enter into descriptive 

 zoology. 



In Botany, orders and families are identical. In Zoology we 

 use the term Family for a group inferior to an order, and equi- 

 valent to the sub-order or tribe in botany. The family con- 



