IX.] THE COMMON FROG. 141 



absorption, and as their respective arteries and veins 

 decrease in size and importance, each ductus botalli 

 increases until at last we have established the six 

 great continuous vessels of the adult frog-. 



We have, then, in the life-history of the frog, a 

 complete transition from the condition of the fish to 

 that of a true air-breathing vertebrate, as regards its 

 circulation. The various conditions herein referred to 

 have, however, an important bearing on the question 

 of the first origin of such structure. 



All higher animals, even the very highest, have the 

 great arteries when they first appear, arranged sub- 

 stantially as in fishes. 



From the common aortic bulb five vessels ascend 

 each side of the neck, and more or fewer of these 

 arteries abort in different classes, the permanent adult 

 condition being arrived at by this circuitous route. 



This argument has commonly been adduced as an 

 argument in favour of the descent of air-breathing 

 animals from more ancient gill-bearing forms, and it 

 is not without weight. 



Nevertheless, it must be borne in mind that the 

 primitive condition in fishes is that of direct con- 

 tinuity between the branchial arteries and veins such 

 as we have seen exists permanently in Lepidosh^en. It 

 is only as development proceeds that each primitive 

 continuous arch becomes broken up into an artery 

 and a vein connected by a network of capillaries. 



Now we can understand the series of unbroken 

 arches in higher animals as the relics of ancestral 

 vessels which divided for gill-circulation and were 

 therefore once of extreme functional importance and 



