CLASSIFICATION IN THE ANIMAL KINGDOM. 21 



haTing laid it down that " in comparing species with each other, for 

 purposes of classification, there are four distinct grounds on which 

 comparison can be made: 1st. ultimate structural, or anatomical 

 resemblance ; 2nd. grade or rank ; 3rd. use or function ; 4th. plan 

 or type ;" proceeds to explain the value and mode of use of each of 

 these, in remarks well deserving attention. I must, however, hold to 

 be very doubtful the opinion of Agassiz, here adopted by Dr. Dawson* 

 that difference of grade and rank is to be specially used for grouping 

 genera into orders. Order is the name employed in natural science 

 to express groups of organisms, next in extent to what are called 

 classes, which were formerly regarded as the primary divisions of the 

 kingdoms of nature ; though, with increasing knowledge, it has been 

 found necessary to interpose sub-kingdoms, or branches. I have not 

 yet been able to appi-eciate the reasons why groups of one degree 

 of comparative extent, should be founded on a different kind of 

 characters, or a different mode of considering them from those of 

 either a higher or lower degree ; and, practically, I must maintain 

 that there is a gradation of ranks in the sub-kingdoms of each 

 kingdom, in the classes of each sub-kingdom, and in the families and 

 Bub-families of each order, as well as in the orders of a class. Nobody 

 doubts that Vertebrata is the highest sub-kingdom of the animal 

 kingdom. Mammalia is universally received as the highest class of 

 Vertebrata. Aves comes next, then Reptilia, and Pisces takes the 

 lowest rank ; whilst those who admit Amphibia, insert it between the 

 last two. iGrade, or rank, is not then us^d, only or chiefly for orders, 

 and neither are the orders usually admitted founded exclusively upon 

 it. On the other hand, when two groups have manifestly a common 

 plan, and corresponding sub-divisions, but differ greatly in grade of 

 development, they are now usually regarded as sub-classes of one 

 class, whilst their corresponding sub-divisions are called orders, a 

 practice entirely at variance with what is here laid down. Dr. 

 Dawson alleges, as an instance of a grave error, arising from the 

 improper application of difference of grade or rank, " the attempt of 

 some naturalists to establish a province or sub-kingdom of Protozoa^ 

 to include all the simplest members of the animal kingdom." I am 

 afraid I fall under my friend's censure in this matter, as being one 

 who deem the admission of Protozoa, as a sub-kingdom, necessary for 

 the right interpretation of the system of nature ; but I must protest 

 against being supposed to defend their separation on the ground of 



