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, 



