CLASSIFICATION IN THE ANIMAL KINGDOM- 25 



Dr. Dawson, -will be in favour of five classes to a sub-kingdom, and 

 five sections under each distinct type as we go on with our sub- 

 division, and looking to the actual fact in respect to what seem the 

 most satisfactory arrangements in various portions of the animal 

 kingdom, I find this view confirmed, the remarkable exception in the 

 sub-divisions of the two lowest branches and, as I think, also in the 

 vegetable kingdom, when properly explained, only making the general 

 law more certain. But since Dr. Dawson has given us the four classes 

 which appear to him good and sufiScient in each of the sub-kingdoms, 

 I will review these in order to hring my own system into fair com- 

 parison with his. In Vertebratahe includes Mammals, Birds, Reptiles 

 and Fishes, omitting Amphibia which he agrees with many others in 

 making an order of Reptilia, considering the distinction to consist 

 chiefly in rank or grade and to be therefore of a secondary kind. It 

 cannot fairly be denied that the remarkably close correspondence of 

 the divisions of Amphibia with those of Reptilia favours this view 

 supposing us to make them, not an order of Reptilia, but a sub-class, 

 an outer circle of corresponding but more rudimentary forms, like the 

 relation of Entozoa to Annulata, but even thus the embryonic and 

 anatomical differences are too strong not to suggest the propriety of 

 their being accounted distinct classes, and the other instances which 

 occur in which, in a natural circle, the third division is terrestrial, 

 the fourth amhphibious, (living partly in water, or near water, from 

 which they derive much of their food), the fifth aquatic, strengthen 

 our expectation of an intermediate class between reptiles and fishes of 

 just such a character as belongs to Amphibia. Dr. Dawson in a note 

 on this point says : " The Amphihia, as Dana well argues on the prin- 

 ciples of cephalisation, are clearly reptiles, because we arrange animals 

 in their mature and not in their embryonic condition, and because the 

 points of reproduction in which Amphibia difPer from ordinary rep- 

 tiles, have relation to an aquatic habit, aud are ordinal or rank charac- 

 ters merely." Elsewhere, also, he objects, and very justly in my 

 opinion, to " basing classification wholly on embryology, or on mere 

 anatomical structure." The truth, I apprehend to be, that in 

 endeavouring to recognise the really distinct types which occur in 

 nature, we employ combinations of various characters, and we suc- 

 ceed so far only as we give its due value to each. There are striking- 

 anatomical differences which are only adaptive modifications in respect 

 to secondary differences of mode of life, though a mere technical 



