THE EVIDENCE OF THE CENTRAL NERVOUS SYSTEM 1 5 



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 fouud, 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 



