88 WILLIAM PATTEN. 



similarity of function or condition. The mere enumeration 

 of the resemblances which my as yet superficial study enables 

 me to point out ought to convince the most sceptical that 

 we are working in the right direction. Is there any group of 

 Invertebrates besides the Arachnids which by any reason- 

 able assumption can be made to resemble Vertebrates in 

 the way that Limulus does ? How far do the comparisons 

 of either Ascidians, Balanoglossus, Nemertians, and Annelids 

 with Vertebrates carry us ? They explain nothing either in 

 Vertebrate anatomy or in phylogeny, and only serve to render 

 the whole problem of the ancestry of the Vertebrates hope- 

 lessly perplexing and obscure. If we accept the Arachnid 

 theory all this is changed, for we can see, dimly perhaps, some 

 way out of the intricate problems bound up in the morphology 

 of the Vertebrate head. It is true many great problems will 

 be left unsolved, but their discussion will be shifted to the 

 vast field of Arthropod morphology, where there is hope of 

 their ultimate solution. And the problems involved are great 

 ones, notwithstanding a dawning tendency to subordinate 

 phylogenetic questions to profound studies on the mechanics 

 of cell division and kindred topics. If the Arachnid theory is 

 true, the Ascidians, Amphioxus, and perhaps Balanoglossus 

 and the Echiuoderms become degenerate phyla, bearing some- 

 what the same relation to the Vertebrates that the parasitic 

 Copepods do to the Arthropods. It will furnish us with a new 

 basis for embryological interpretation, especially regarding the 

 problems connected with the formation of germ layers. If 

 the Arachnid theory is true, many current views on phylogeny, 

 ontogeny, and important problems in comparative anatomy are 

 based on false conceptions, and must be revised. That this is 

 no exaggeration is obvious from the objections most frequently 

 urged against the Arachnid theory, ones I had not anticipated 

 would have any weight. Instead of considering the question 

 on its own merits, it has been objected that " it was contrary 

 to all our preconceived ideas," and " it is quite impossible to 

 conceive highly specialized animals like Arachnids giving rise 

 to such highly specialized animals as Vertebrates." 1 do not 



