PART V 

 CHAPTER XLI1I 



CONCLUSIONS 



Not the least of the interests aroused by the study of Verte- 

 brates is due to the fact that they form a group which lends itself 

 perhaps better than any to a consideration of general principles 

 and matters of wide importance. This is largely because, 

 although imperfect, present knowledge covers a considerable 

 amount of the results of vertebrate evolution, and still more 

 because between the most widely separated members of the 

 group, between Amphioxus and man, there is sufficient simi- 

 larity in plan of structure to enable comparisons to be made 

 with advantage. Comparative Anatomy as an intellectual 

 weapon is the more satisfactory when the number of corre- 

 spondences of kind which can be established is great, regardless 

 of course of matters of detail. So it is not astonishing that the 

 Anatomy of, for example, Nematodes and Echinoderms when 

 compared should be less fertile in conclusions of general 

 interest than a comparison between Vertebrates as distant 

 from one another as are fish and mammals. From the fact 

 of the general homogeneity of the group as a whole, the varia- 

 tions to be observed in different vertebrates become all the more 

 interesting. 



It is very striking to find organs such as notochord, nerve- 

 tube, dorsal and ventral nerve-roots, essentially the same in 

 Amphioxus and man, but the most striking case of homologous 

 organs is that of the thyroid. From the endostyle of 

 Amphioxus, through Petromyzon with its tell-tale Ammocoete 

 larva, to all the Craniates, the chain is complete, and not the 



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