318 THE STRUCTURAL EVIDENCE OF EVOLUTION". 



branchii to have acquired its present characters by a process of 

 degeneration. The origin of the Vertebrata is as yet entirely un- 

 known, Kowalevsky deriving them from the Ascidians, and 

 Semper from the Annelida. The above results I have embodied 

 in the following phylogenetic diagram : 



Aves Mammalia 

 \ / 



\ / 



Reptilia 



/ 

 / 



C Teleostomi* Batrachia 



I I \ 



Pisces -l Selachii Ichthyotomi Dipnoi 



Holocephali 

 Leptocardii 



Marsipobranchi 



Accepting this j)hylogeny, it becomes possible to determine 

 the course of development, first, of the whole series ; and, secondly, 

 of the contents of each class taken by itself. I will first consider 

 the direction of the evolution of the Vertebrata as a whole. 



IT. THE VEKTEBEATE LINE. 



The Vertebrata exhibit the most unmistakable gradation in 

 the characters of the circulatory system, f It has long been the 

 custom to define the classes by means of these characters, taken 

 in connection with those of the skeleton. Commencing in the 

 Leptocardii with the simple tube, we have two chambers in the 

 Marsipobranchii and fishes ; three in the Batrachia and Reptilia ; 

 and four in the Aves and Mammalia. The aorta-roots commence 

 as numerous pairs of branchial arteries in the Leptocardii ; we 

 see seven in the Marsipobranchi, five in the fishes (with number 

 reduced in some) ; four and three in Batrachia, where they gener- 

 ally cease to perform branchial functions ; two and one on each 

 side in Reptilia ; the right-hand one in birds, and the left-hand 

 one in Mammalia. This order is clearly an ascending one through- 

 out. It consists of, first, a transition from adaptation to an aquatic, 

 to an aerial respiration ; and, second, an increase in the power to 

 aerate and distribute a circulating fluid of increased quantity, 



* In the original I used the name Hyopomata for tiiis division, but Owen's name 

 Teleostomi is prior. (Ed. 1880.) 



\ See " Origin of Genera," 1868, p. 20, for a table of the characters of the circu- 

 latory system. 



