Zoological Position. 17 



segmented type of limb with a full complement of muscles, and 

 originally a pentadactyl, plantigrade foot, for support of the body 

 and for locomotion. The regional differentiation of the vertebral 

 column, especially the mobiHty of the neck, the free occipital 

 articulation, and the definition of the sacrum, the latter associated 

 with, the elaboration of the pelvic girdle, are all features of general 

 significance in the terrestrial vertebrates. 



Finally, the rabbit agrees with other members of the phylum 

 Chordata in the possession of a ground-plan underlying the most 

 general features of its organs, and the position, arrangement, and 

 plan of development of its organ-systems. All Vertebrata or 

 back-boned animals possess an axial skeleton formed by the 

 segmented vertebral column. In a very comprehensive way they 

 possess as chordates a still more fundamental axial support, the 

 notochord, the latter being an embryonic structure except in the 

 lowest chordates. In a more restricted sense, as Craniota, they 

 possess an organized head region with differentiated bi'ain, special 

 sense organs, and enclosing primary skull. They possess a series 

 of branchial (branchiomeric) structures, appearing either in the 

 adult condition, as in fishes, or as part of the underlying plan in the 

 embryonic condition; and they add to their general features in 

 the arrangement of the organ-systems the further feature of 

 transverse segmentation (metamerism) of a considerable portion 

 of the body. 



These facts may be set down in tabular form, as indicated below. 

 A similar plan can be constructed for any group of organisms, but 

 whether it constitutes a natural or an artificial classification depends 

 on whether or not it is based upon an actual study of the affinities 

 of the organisms concerned. A natural classification should show 

 at a glance not only what the relative importance of any particular 

 character may be, but also how it stands in the scale of specializa- 

 tion. For example the placental stage of vertebrate develop- 

 ment, i.e. the development of the placenta itself in the highest 

 stage of vertebrate evolution, is the culmination of a series of 

 arrangements for the care of eggs and young, and further the 

 adherence of human structure in the vast majority of features to 

 type of higher mammals is similarly expressed by the fact that man 

 -is also a placental mammal. 



