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 mobility 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 
signifiicance 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 
segmenied 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 brain, 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, 7.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. 
