BLOOD SYSTEM. .QI 



stages from a right-turning to a left-turning arch would 

 be unsuitable. We will show that they both came from 

 reptiles, but by different routes. 



Again we have drawn attention to the strange fact 

 that in reptiles (e. g., the lizard) there are three arches 

 on each side — six in all. Why six arches ? Surely, one 

 arch is enough: for birds and mammals have but one, 

 and they have the most perfect circulation of all. The 

 key to the mystery is found in the circulation of fishes. 

 Fishes have three or four or five arches on each side, but 

 the reason in their case is obvious. They are the gill 

 arches. They are absolutely necessary for the aeration 

 of the blood. If the reptiles came by modification from 

 fishes, then the explanation of their numerous arches is 

 plain. They are the remnants of ancestral gill arches. 

 Some of the highest of fishes, the Dipnoi, have only 

 three arches on each side like the lizard. These fishes 

 also breathe by lungs as well as by gills. Now, sup- 

 pose in successive generations the lungs of Dipnoi to 

 increase in efficiency, and the gills to dwindle and dry 

 up, it is evident that exactly the structure found in the 

 lizard would remain. 



That exactly this did take place in the history of the 

 evolution of vertebrates is proved by the fact that every 

 step is found now among fishes, amphibians, and reptiles, 

 and furthermore that the same change takes place now 

 in the individual history of every one of the higher am- 

 phibians, as, for example, the frog. We have already 

 traced the successive steps, through teleosts, ganoids, 

 Dipnoi, perennibranchiate amphibians, caducibranchiate 

 amphibians, to reptiles. We wish now to trace the same 

 change in the individual development of a frog. 



The frog, as we all know, is at first a tadpole ; without 

 feet, swimming only by the tail; without lungs or air 

 breathing — in fact, breathing only water by gills. In this 



