The Breath of a Bird 181 
is the case in all the warm-blooded creatures above it, 
the heart is nearer the under side of the body—the breast 
—than near the back. And herein les an important 
difference between the two great divisions of the Ani- 
mal Kingdom, vertebrates and invertebrates,—the former 
always having the heart near the breast, while in the back- 
boneless organisms it is near the back. 
The heart of a fish is fairly concentrated and muscu- 
lar, but the blood which passes through it is but an im- 
pure and sluggish stream. In reptiles both pure and 
impure blood is found in the heart, but they mingle, and 
thus half destroy the purifying action of the lungs. This 
explains why these animals are cold-blooded, and also 
accounts for their usual lethargic disposition and low 
mental plane of life. 
In crocodiles we find a significant condition. There 
are four chambers in the heart, as in mammals and in 
birds, but this avails nothing; for, leading from the heart 
are two arteries instead of one, and where these cross 
each other there is a tiny aperture—a small opening in 
the partition which allows the impure blood to leak into 
the stream of pure, red blood, and so a crocodile is only 
a crocodile, although evolution has lifted his heart al- 
most to a level with birds and the warm-blooded ani- 
mals. If this tiny hole could become closed, and the 
two streams of blood be kept separate, the eyes of the 
crocodile would brighten, his activity increase many fold, 
and in fact his entire plane of life would be changed. 
I have thus briefly reviewed the heart in the lower 
vertebrates in order to give a more vivid idea of this organ 
