144 On the Evolution of the Vertebrata, [February, 
there remain the following: the Pisces, Batrachia, Reptilia, 
Aves and Mammalia. 
The Mammalia have been traced to the theromorphous reptiles 
by the Monotremata. The birds, some of them at least, appear 
to have been derived from the Dinosaurian reptiles. The reptiles 
in their primary representative order, the Theromorpha, have 
been probably derived from the rhachitomous Batrachia. The 
Batrachia have originated from the sub-class of fishes, the Dipnoi, 
though not from any known form. I have shown that the true 
fishes or Hyopomata have descended from an order of sharks, the 
Ichthyotomi, which possess characters of the Dipnoi also. The 
origin of the sharks remains entirely obscure, as does also that of 
the Pisces as a whole. Dohrn believes the Marsipobranchii to 
have acquired its present characters by a process of degeneration. 
The origin of the Vertebrata is as yet entirely unknown, Kowal- 
evsky deriving them from the Ascidians, and Semper from the 
Annelida. The above results I have embodied in the folowing 
— diagram : 
Aves Mammalia 
Reptilia 
Hyopomata Batrachia 
Pisces | Selachii Ichthyotomi Dipnoi 
| 
Holocephali 
Marsipobranchi 
Leptocardii 
Accepting this phylogeny, it becomes possible to determine 
the course of development first of the whole series ; and sec- 
ondly of the contents of each class taken by itself. I will first 
consider the direction of the evolution of the Vertebrata as a 
whole. 
II. THE VERTEBRATE LINE. 
The Vertebrata exhibit the most unmistakable gradation in 
the characters of the circulatory system.” It has long been the 
1 Proceedings Am. Phil, Soc., 1884, . 585. 
2See Origin of Genera, 1868, p. 20, for a table of the characters of the circulatory 
. System. 
