l6 PII<X.'EED1NGS OK THK 



Those changes about the uature of which all zoologists are agreed, 

 such as the rt'hitionship of an abei-raut genus to the typical form 

 of the family or ordi-r to wliich it belongs. Thus no one doubts 

 that the Ili'riiiit Crab is defscended from a normal Lobster or 

 Chiftojiterus from a normal Annelid. The changes involved in the 

 descent of such forms from the more normal types give us the 

 only rules we can have to guide us vvlien we attempt the more 

 dithcidt task of passing from one phylum to another. 



Now Dr. Gaskell, in assuming that Vertebrates are descended 

 from some Pala^ostracan type of Arthropod of which the only 

 survivor is Limulus, is obliged to reconstruct the entire animal, 

 leaving only the central nervous system standing. We are asked 

 to believe that the original alimentary canal has become the neural 

 canal, and that a new alimentary canal has developed from the 

 skin of the ventral surface of the body. No precedent for such a 

 change can be gathered from any of the data I have mentioned above. 



Again, the skin of the lower Vertebrates is ciliated, and this is 

 most undoubtedly a primitive condition seeing how widely it is 

 spread amongst the lower groups in the Animal Kingdom. No 

 Arthropod* is ciliated at any time of its existence: its whole 

 organisation is dominated by the tendency to form thick chitinous 

 cuticle. AVe have to suppose that this tejidency, which is spread 

 throughout Arthropoda from the highest to the lowest, has been 

 overcome and that a reversion to a primitive soft ciliated ectoderm 

 has been accomplished. No precedent for such a change can be 

 gathered from the entire Animal Kingdom. It is no answer to this 

 to show that in Ainmoccptes and one or two other cases a thin 

 exterior cuticle is developed on certain parts of the skin— for it 

 is the normal sequence of things that a cuticle should succeed to 

 a ciliated skin as a secondary change, but the change in the reverse 

 direction is absolutely without precedent. 



The eyes of Vertebrates, or, to speak more correctly, their 

 retina?, are lateral pockets of the walls of the neural canal — which 

 we are told to regard as the old alimentary canal. The eyes of 

 Arthropoda are, without exception, modifications of the external 

 skin. Are the lateral eyes ot the two groujjs homologous or are 

 they not ? If they are homologous, how is their different origin 

 explained ? Dr. Gaskell figures a section of Artemia in which 

 one of tiie liver saccules is in close contact with the lower layer 

 of the eye. He hints that perhaps part of the eye is developed 

 from the epithelium of the liver saccule, but this is in flat con- 

 tradiction to the work of eveiy zoologist who has examined their 

 development. If the eyes in the two cases are not homologous, 

 why did the Arachnid ancestor of Vertebrates give up its external 

 eyes and develop a new pair from its old alimentary canal ? To 

 say that there is no precedent for such a change is to put it mildly. 



* I hardly think it necepsary to refer to the cihation of the genital ducts of 

 Ptiripaiua, the only exception to this rule, since Peripafus is hardly as 3'et an 

 Arthropod. 



