ON THE HYPOTHESIS OF EVOLUTION. 133 



organs are most complex or specialized. The nervous ring is 

 almost obliterated by a great enlargement of its usual ganglia, 

 thus become a brain, which is succeeded by a long axis on the 

 upper side of the body. This and other points define the Ver- 

 tebrata. 



Plans of structure, independent of the simplicity or perfection 

 of the special arrangement or structure of organs, also define 

 these great groups. Thus the Protozoa present a spiral, the 

 Coelenterata a radiate, the Echinodermata a bilateral radiate plan. 

 The Articulates are a series of external rings, each in one or more 

 respects repeating the others. The Molluscs are a sac, while a 

 ring above a ring, joined together by a solid center-piece, repre- 

 sents the plan of each of the many segments of the Vertebrates 

 which give the members of that branch their form. 



These bulwarks of distinction of animal types are entered into 

 here simply because they are the most inviolable and radical of 

 those with which we have to deal, and to give the anti-develop- 

 mentalist the best foothold for his position. I will only allude to 

 the relations of their points of approach as these are affected by 

 considerations afterward introduced. 



The Vertebrates approach the Molluscs closely at the lowest 

 extreme of the former and higher of the latter. The lamprey 

 eels of the one possess several characters in common with the 

 cuttle-fish or squids of the latter. The amphioxus is called the 

 lowest Vertebrate, and, though it is nothing else, the definition of 

 the division must be altered to receive it ; it has no brain ! 



The lowest forms of the Molluscs and Articulates are scarcely 

 distinguishable from each other, so far as adhesion to the "plan " 

 is concerned, and some of the latter division are very near certain 

 Echinodermata. As we approach the boundary-lines of the two 

 lowest divisions, the approaches become equally close. 



More instructive is the evidence of the relation of the subordi- 

 nate classes of any one of these divisions. The conditions of 

 those organs or parts which define classes exhibit a regular rela- 

 tion, commencing with simplicity and ending with complication ; 

 first associated with weak exhibitions of the highest functions of 

 the nervous system — at the last displaying the most exalted traits 

 found in the series. 



For example : In the classes of Vertebrates we find the lowest 

 nervous system presents great simplicity — the brain can not be 

 recognized ; next (in lampreys), the end of the nervous axis is 



