The Theory of Evolution 67 
walled muscular tube below the pharynx; the blood enters at 
its posterior end, flows forward and out at the anterior end 
into a blood vessel that sends smaller vessels up through the 
gill-arches to the dorsal side. 
In the amphibia the heart is a tube, so twisted on itself that 
the original posterior end is carried forward to the anterior 
end, and this part, the auricle, is divided lengthwise by a 
partition into a right and a’‘left side. In the reptiles the 
ventricle is also partially separated into two chambers, com- 
pletely. so in the crocodiles. In birds and mammals the 
auricular and ventricular septa are complete in the adult, and 
the ventral aorta that carries the blood forward from the 
heart is completely divided into two vessels, one of which now 
carries blood to the lungs. When we examine the develop- 
ment of the heart of a mammal, or of a bird, we find some- 
thing like a parallel series of stages, apparently resembling 
conditions found in the different groups just described. The 
heart is, at first, a straight tube, it then bends on itself, and a 
constriction separates the auricular part from the ventricular, 
and another the ventricular from the ventral aorta. Vertical 
longitudinal partitions then arise, one of which separates the 
auricle into two parts, and another the ventricle into two 
parts, and a third divides the primitive aorta into two parts. 
In the early stages all the blood passes from the single 
ventral aorta through the gill-arches to the dorsal side, and it 
is only after the appearance of the lung-system that the gill- 
system is largely obliterated. 
We find here, then, a sort of parallel, provided we do not 
inquire too particularly into details. This comparison may be 
justified, at least so far that the circulation is at first through 
the arches and is later partially replaced by the double cir- 
culation, the systemic and the pulmonary. 
A few other cases may also be added. The proverbial 
absence of teeth in birds applies only to the adult condition, 
for, as first shown by Geoffroy Saint-Hilaire, four thickenings, 
