N 
LARGE SIZE OF MAMMALIA CHAP. 
as a biting organ. The term highest, however, includes increased 
complexity as well as simplification, the two series of modifica- 
tions being interwoven to form a more efficient organism. It 
cannot be doubted that the increased complexity of the brain of 
mammals raises them in the scale, as does also the complex and 
delicately adjusted series of bonelets which form the organ for 
the transmission of sound to the internal ear. The separation of 
the cavity containing the lungs, and the investment of the parti- 
tion so formed with muscular fibres, renders the action of the 
lungs more effective; and there are other instances among the 
Mammalia of greater complexity of the various parts and organs 
of the body when compared with lower forms, which help to 
justify the term “highest ” generally applied to these creatures. 
Complexity and finish of structure are often accompanied by 
large size; and the Mammalia are, on the whole, larger than any 
other Vertebrates, and also contain the most colossal species. The 
huge Dinosaurs of the Mesozoic epoch, though among the largest 
of animals, are exceeded by the Whales; and the latter group 
includes the mightiest creature that exists or has ever existed, 
the eighty-five-feet-long Sibbald’s Rorqual. Confining ourselves 
rigidly to facts, and avoiding all theorising on the possible 
relation between complexity and nicety of build and the capacity 
for increase in bulk, it is plain from the history of more than 
one group of mammals that crease in bulk accompanies specialis- 
ation of structure. The huge Dinocerata when compared with 
the ancestral Pantolambda teach us this, as do many similar 
examples. Within the mammalian group, as in the case of other 
Vertebrates, difference of size has a certain rough correspondence 
with difference of habitat. The Whales not only contain the 
largest of animals, but their average size is great; so too with 
the equally aquatic Sirenia and very aquatic Pinnipedia. Here 
the support offered by the water and the consequent decreased 
need for muscular power to neutralise the effects of gravity 
permit of an increase in bulk. Purely terrestrial animals come 
next; and finally arboreal, and, still more, “ flying” mammals 
are of small size, since the maintenance of the position when 
moving and feeding needs enormous muscular effort. 
The Mammals are more easily to be separated from the 
Vertebrates lying lower in the series than any of the latter are 
from each other in ascending order. A large number of char- 
