39 o PHYSIOLOGY AND MORPHOLOGY OF ANIMALS. 

 BEARING OF SOME OF THESE FACTS ON EVOLUTION. 



i. Heart Structure.— There is abundant evidence, 

 both in the taxonomic series (animal scale) and the 

 embryonic series, that the heart has been gradually 

 formed by a process of evolution, for nearly all the 

 stages may be traced from the simplest pulsating organ 

 to the complex four-chambered valvulated structure of 

 the higher animals. The steps are briefly as follows : 



(i) First there is a simple pulsating dorsal vessel. This 

 stage is found in many invertebrates, and even among 

 vertebrates in the amphioxus. 



(2) Then comes a one-chambered heart. This is 

 found in many invertebrates. 



(3) Then comes the two-chambered heart of many 

 invertebrates, and of fishes among vertebrates. 



(4) Then the three-chambered heart of the typical 

 reptile and amphibian. 



(5) Then the imperfect four-chambered heart of ser- 

 pents, the two sides of the heart still connected. 



(6) Then the perfect four-chambered heart of croco- 

 dilians ; but even yet a connection (ductus arteriosus) by 

 which the blood to the tissues is mixed, and therefore 

 the circulation is not completely double. 



(7) The final step is the perfect four-chambered heart 

 and complete double circulation of birds and mammals. 



Nearly all these steps are found also in the embry- 

 onic development of mammals and even of man. 



2. Origin of Aortic Arches. — But by far the most 

 interesting question in this regard is the origin and 

 meaning of aortic arches. 



We have already drawn attention to the fact that the 

 aortic arch in the bird turns to the right, and not to the 

 left, as in mammals. This shows that mammals did not 

 come from birds by modification, for the intermediate 



