942 The American Naturalist. November, 
The brain arose, by the present supposition, from the gang- 
lia of the “ ventral ” chain of Annelids. Even in Articulata, 
in the Crustacea and Arachnida, we encounter a tendency of 
the thoracic ganglia to make a brain. If the brain did thus 
arise, its enlargement would cause, as we see in the Vertebrate 
embryo, the brain to expand forwards, and since there is no 
corresponding growth on the hemal side, this expansion would 
cause the projecting head to bend over towards the hemal 
(Annelid dorsal, or Vertebrate ventral) side, and thus carry 
the mouth into a new position, and at the same time prevent 
the eyes and praeoral ganglia from meeting in the median 
line and force the visual organs into lateral positions. We 
thus reach a natural and simple explanation of the difference 
between the Annelid and Vertebrate head. The conception 
brought forward is favored by the conditions observed in 
Myxine and Petromyzon, for the eyes in these forms are 
small, in Myxine without a lens. The large size of the eyes is 
a subsequent acquisition of the Gnathostomes. By my hypo- 
thesis we should expect the evolution of the brain to precede 
that of the eye in Vertebrates. Finally, the vertebrate brain, 
by our supposition, was segmented originally throughout, and 
that this idea is correct is indicated by the trend of recent 
opinions formed by investigators of the cephalic neuromeres, 
and the morphology of the cephalic nerves in the Verte- 
brata. 
The series of considerations outlined justify the hypothesis 
presented, namely, the Vertebrate head was evolved from the 
Annelid head. The three principal factors in this evolution 
were: 1. The formation of the vertebrate brain from several 
of the post-oral ganglia of the ventral chain. 2. The preser- 
vation of the ectodermal mouth-gut (Vorderdarm) of Annelids 
as the naso-hypophysal invagination. 3. The conversion of 
the visual apparatus and supra-oesophageal ganglia, which 
are prevented from fusing in the median line by the enlarge- 
ment of the vertebrate brain, into the vertebrate eyes (retina), 
the oesophageal commissures persisting as optic nerves. 
If this hypothesis be ultimately verified and shown to be 10 
full accordance with the facts, it will obviate those difficulties 
