496 THE ORIGIN OF VERTEBRATES 



the vertebrate organs is absolutely hypothetical, no clue is given in 

 Balanoglossus, not even to the segmented nature of the vertebrate. 

 The same holds good with the evidence from Embryology and from 

 Palaeontology. I have pointed out how strongly the evidence in both 

 cases confirms that of Comparative Anatomy. In neither case is the 

 strength of the evidence for Balanoglossus in the slightest degree 

 comparable. In Embryology an attempt has been made to compare 

 the origin of the ccelom in Amphioxus and in Balanoglossus. In 

 Palaeontology there is nothing, only an assumption that in the 

 Cambrian and Lower Silurian times a whole series of animals were 

 evolved between Balanoglossus and the earliest armoured fishes, which 

 have left no trace, although they were able to hold their own against 

 the dominant Palaeostracan race. The strangeness of this conception 

 is that, when they do appear, they are fully armoured, as in Pteraspis 

 and Cephalaspis, and it is extremely hard luck for the believers in 

 the Balanoglossus theory that no intermediate less armoured forms 

 have been found, especially in consideration of the fact that the 

 theory of the origin from the Palaeostracan does not require such 

 intermediate forms, but finds that those already discovered exactly 

 fulfil its requirements. 



One difficulty in the way of accepting the theory which I have 

 advocated is perhaps the existence of the Tunicata. I cannot see 

 that they show any affinities to the Arthropoda, and yet they are 

 looked upon as allied to the Vertebrata. I can only conclude that 

 both they and Amphioxus arose late, after the vertebrate stock had 

 become well established, so that in their degenerated condition they 

 "ive indications of their vertebrate ancestry and not of their more 

 remote arthropod ancestry. 



In conclusion, the way in which vertebrates arose on the earth as 

 suggested in this book carries with it many important far-reaching 

 conclusions with respect to the whole problem of Evolution. 



When the study of Embryology began, great hopes were entertained 

 that by its means it would be possible to discover the pedigree of 

 every group of animals, and for this end all the stages of development 

 in all groups of animals were sought for and, as far as possible, 

 studied. It was soon found, however, that the interpretation of 

 what was seen was so difficult, as to give rise to all manner of views, 

 depending upon the idiosyncracy of the observer. At his will he 

 decided whether any appearance was cuenogenetic or palingenetic, 



