1 6 THE ORIGIN OF VERTEBRATES 



organ being considered by them as the great characteristic of the 

 vertebrate ; indeed, so much is this the case, that a large number 

 of zoologists speak now of Chordata rather than of Vertebrata, and 

 in order to emphasize their position follow Bateson, and speak of the 

 Tunicata as Uro-chordata, of Amphioxus as Cephalo-chordata, of the 

 Enteropneusta as Hemi-chordata, and even of Actinotrocha (to use 

 Masterman's term), as Diplo-chordata. 



The upholders of this theory lay no stress on the nature of the 

 central nervous system in vertebrates, they are essentially zoologists 

 who have made a special study of the invertebrate rather than of 

 the vertebrate. 



Of these two methods of investigating the problem, it must be 

 conceded that the former is more likely to give reliable results. 

 By putting the vertebrate to the question in every possible way, by 

 studying its anatomy and physiology, both gross and minute, by 

 inquiring into its past history, we can reasonably hope to get a 

 clue to its origin, but by no amount of investigation can we tell 

 with any certainty what will be its future fate ; we can only guess 

 and prophesy in an uncertain and hesitating manner. So it must be 

 with any theory of the origin of vertebrates, based on the study of 

 one or other invertebrate group. Such theory must partake rather 

 of the nature of prophecy than of deduction, and can only be placed 

 on a firm basis when it so happens that the investigation of the 

 vertebrate points irresistibly to its origin from the same group ; in 

 fact, " never prophesy unless you know." 



The first principle, then, I would lay down is this : In order to 

 find out the origin of vertebrates, inquire, in the first place, of the 

 vertebrate itself. 



Impoetance of the Central Nervous System. 



Does the history of evolution pick out any particular organ or 

 group of organs as more necessary than another for upward progress ? 

 If so, it is upon that organ or group of organs that special stress must 

 be laid. 



Since Darwin wrote the " Origin of Species," and laid down that 

 the law of the ' survival of the fittest ' is the factor upon which evolu- 

 tion depends, it has gradually dawned upon the scientific mind that 

 ' the iittest ' may be produced in two diametrically opposite ways : 



