1864.] DR. DAWSON ON CLASSIFICATION OF ANIMALS. 249 



kind. Of errors of these kinds still current, I may instance the 

 atteni'pt of some naturalists to establish a province or sub-kingdom of 

 Protozoa, to include all the simplest members of the Animal King- 

 dom, and the separation of the Entozoa or intestinal worms from 

 the other worms as a distinct class. The classification in Owen's 

 '' Lectures on the Invertebrate Animals," which I have long used 

 with advantage as a text-book, is defective in some parts in this 

 respect. 



There are two kinds of investigation much used in classifica- 

 tion, which more especially develope the idea of grade or rank 

 among animals. One is that of embryology, or the development 

 of animals from the ovum. Another is that of cephalisation, or 

 the development of the head and organs connected therewith. 

 Both of these are of great importance, but, on the principles 

 above stated, they aid us chiefly in referring animals to their 

 Orders. Other limitations of the criterion of grade or rank will 

 appear when we arrive at the consideration of Classes. 



3. function or Use. — In difierent animals we often find the 

 same use served by difierent kinds of organs, as, for instance, the 

 wing of a bird and the wing of an insect, which, though both 

 used for flying, are constructed in very difi"erent ways. It would 

 lead us astray were we to arrange animals primarily on this ground : 

 for instance, if we were to group together fishes and Crustacea 

 because both swim ; or birds and insects, because both fiy. Again, 

 in difierent groups of animals, certain functions and the organs 

 which subserve them are greatly developed in comparison with 

 others. For example, the enormous reproductive power of fishes, 

 or the remarkable development of the locomotive organs in birds, as 

 compared with other vertebrates. This consideration is not ap- 

 plicable in our primary division of animals, but it constitutes the 

 principal ground on which naturalists have based the secondary 

 divisions or Classes; and it serves also to indicate the analogies 

 between the corresponding members of difierent primary groups, 

 as, for instance, of the birds in one group to the insects in 

 another. 



4. Plan or Type. — Under this head we consider the similarity 

 of construction in different animals or organs, without regard to 

 uses. We say, for example, that the wing of the bird and the bat, 

 the paddle of the whale, and the fore-leg of the dog, are similar 

 in type or homologous to each other, because they are made up of 

 similar sets of bones. They are modifications of one general plan 



