336 MR. W. WOODLAND ON THE [ Apr. 21, 
and enquiry confirms the supposition. Though, in the above 
account of mammalian locomotion, most stress has been laid upon 
the impulsive elevations of the body and resulting strains on the 
attachments of organs, yet it must not on that account be inferred 
that on descent of either half of the trunk the shock consequent 
on contact of the limbs with the earth (equal in intensity to the 
elevatory impulse) is negligible. (See above in case of Kangaroo 
in which it is taken into account.) Such shocks have the effect 
of causing those viscera which are closely adherent to the mass of 
the body either to exert a considerable pressure on structures 
anterior to them or to be dislodged from theix normal position ; 
and if in either case such a result is to be avoided, organs of 
support must be developed. In mammals, the lungs together with 
the heart occupy the anterior portion of the ccelomic cavity, and 
behind these are situated the liver, stomach, and intestines, these 
together constituting a considerable mass. These massive organs, 
unless prevented, would on each contact of the limbs with the 
earth exert great pressure on the fragile compressible lungs 
immediately anterior to them. Hence, in order to obviate 
ensuing derangements, we find in all mammals, and in many other 
terrestrial animals, a stout partition separating the cavity of the 
liver and gut from the cavity of the lungs—a structure which can 
only have been originated by natural selection. Evidence sup- 
porting the conclusion that the diaphragm arose as an adaptation 
to the forward pressure of the liver and gastric mass, is not only to 
be found in the fact that the diaphragm is convex anteriorly, but 
also in that the convexity is, cewteris paribus, proportional to the 
impulsiveness of the animal’s activity and to the mass of the 
liver and gut. In herbivores the mass of the gut is greater than 
in carnivores; on the other hand, ‘‘ carnivorous (fat-eating) animals 
generally possess a larger liver than herbivores” (Wredersheim), 
so that these respective characters tend to defeat comparison *. 
But in the Perissodactyla we find a fairly large liver (considerably 
larger than in Artiodactyla), a long herbivoran gut, and a high 
degree of impulsiveness, the combination of which on our hypo- 
thesis should be correlated with a highly convex diaphragm—an 
inference which proves correct. Owen remarks that “in the 
perissodactyle Ungulates, in which the movable ribs are numerous 
and continued to near the pelvis, the diaphragm is also extensive 
and much arched towards the thorax ”—indicating that the con- 
vexity of the diaphragm is markedly above the normal. And, 
doubtless, other illustrations are adducible. Additional evidence 
as to the primary function of the diaphragm is, moreover, afforded 
by the case of the Struthiones (and allies) which have adopted 
the mammalian mode of locomotion. In these birds there exists 
a “ well-marked diaphragm forming a partition which divides the 
thoracic cavity into two parts, one posterior and small containing 
the lungs, and the other anterior and large containing the heart 
1 The small capacity of the herbivoran thoracic cavity as compared with that of 
the carnivora must be remembered in this connection. 
