THE EVIDENCE OF THE CENTRAL NERVOUS SYSTEM 15 
The discovery that the pineal body was originally an eye, or, 
rather, a pair of eyes, has perhaps more than anything else proved 
the impossibility of accepting this reversal of surfaces as an explana- 
tion of the genesis of the vertebrate from the annelid group. For 
whereas a pair of eyes close to the mid-dorsal line is not only likely 
enough, but is actually found to exist among large numbers of 
arthropods, both living and extinct, a pair of eyes situated close 
to the mid-ventral line near the mouth is not only unheard of in 
nature, but so improbable as to render impossible the theory which 
necessitates such a position. 
Yet this very discovery gives the strongest possible additional 
support to the close identity in the plan of the central nervous 
system of vertebrate and appendiculate. 
A truly paradoxical situation! The very discovery which may 
almost be said to prove the truth of the hypothesis, is the very one 
which has done most to discredit it, because in the minds of its 
authors the only possible solution of the transition from the one 
group to the other was by means of the reversal of surfaces. 
Still, as already said, even if the theory advanced to explain the 
facts be discredited, the facts remain the same; and still to this day 
an explanation is required as to why such extraordinary resemblances 
should exist between the two nervous systems, unless there is a 
genetic connection between the two groups of animals. An ex- 
planation may still be found, and must be diligently sought for, 
which shall take into account the strong evidence of this relation- 
ship between the two groups, and yet not necessitate any reversal 
of surfaces. It is the object of this book to consider the possibility 
of such an explanation. 
What are the lines of investigation most likely to meet with 
success? Is it possible to lay down any laws of evolution? It 
is instructive to consider the nature of the investigations which 
have led to the two theories just mentioned, for the fundamental 
starting-point is remarkably different in the two cases. The one 
theory is based upon the study of the vertebrate itself, and especially 
of its central nervous system, and its supporters and upholders have 
been and are essentially anatomists, whose chief study is that of 
vertebrate and human anatomy. The other theory is based upon the 
study of the invertebrate, and consists especially of an attempt to 
find in the invertebrate some structure resembling a notochord, such 
