General Considerations. xix 



contemplation of the subjects here treated of with reference to 

 their bearings upon Classification, is calculated to impress upon 

 the mind ; and these it may be well to state in a few words. 



After some study of the details of Comparative Anatomy we 

 begin to see that the animal kingdom is divisible into a certain 

 number of Sub-kingdoms accordingly as the various structures or 

 organs subserving the functions of animal and vegetable life re- 

 spectively are combined with, separated from, or otherwise arranged 

 relatively to, each other. It is, in the next place, easy to see that 

 whilst in the Sub-kingdom Vertebrata, motor, nervous, vascular, 

 visceral, and perivisceral systems all exist in specialized and differ- 

 entiated forms, no such ' division of labour ' is recognisable in the 

 structural arrangements of the Sub-kingdom Protozoa ; and that, 

 by a further and detailed reference to the principle just mentioned, 

 we are justified in speaking of the one as the highest and the others 

 as the lowest of the seven animal Sub-kingdoms. It is not easy, 

 however, to assign to the three Sub-kingdoms known as Mollusca, 

 Arthropoda, and Echinodermata their relative rank inter se ; and 

 the Sub-kingdom Vermes would appear to underlie each and all of 

 the three obliquely, rather than to be subordinated to any one of 

 them in particular. The Coelenterata, finally, are approximated to 

 the Protozoa by the low degree to which specialization has been 

 carried out in their organization ; but they form a more than 

 ordinarily well-circumscribed group, which we are in no way 

 justified in regarding as forming a transitional stage intermediate 

 between the Protozoa and the other Sub-kingdoms, from which 

 latter it lies far apart. 



Within the limits of each Sub-kingdom the differentiation of 

 organs, by the assignment of them to the more or less exclusive 

 performance of particular functions, is very often carried out in the 

 different classes to such a different extent as to allow us to speak 

 without violence and without hesitation of such classes as being 

 higher or lower in the scale of existence. Elevation in the scale 

 of life is indirectly entailed in Sub-kingdoms which possess air- 

 breathing representatives, as aerial respiration renders possible a 

 greater activity of function than an organism differing in this, 

 though similarly constituted in all other particulars, can put forth ; 

 whilst the special habit of parasitism, which often renders not 

 merely single organs but even whole systems superfluous, and is 



