THE ORIGIN OF VE:RTF.BRATES. 1 6/ 



that adorn the book do much to help one to reahse, by means of 

 comparison, both the resemblances and the differences which 

 appear respectively in analogous and in homologous organs of 

 the selected representatives of the arthropod and chordate 

 phyla. From these one can gather with intense interest how 

 Dr. Gaskell seeks to establish a case which is certainly much 

 stronger than anyone could have anticipated from a cursory 

 view of the position. 



Thus, assuming that the original cephalic stomach had 

 ceased, or would shortly cease, to perform its functions, he 

 shows how the nervous ganglia which constituted the brain ex- 

 tended round it until it converted itself into what was merely 

 the third ventricle; while the spinal marrow built itself round 

 the original alimentary canal, and the gullet became a closed 

 sac or slight invagination of the brain ( fundibulum). 



This involves the proposition that a new gut or alimentary 

 canal had to be formed. This, the author admits, appears at 

 first sight too great an assumption to make, but justifies it by 

 a comparison of what takes place (as already shown) in the 

 lurijg-fish when it uses its swim-bladder as a lung, and allows 

 its gills to become functionless. The new alimentary system. 

 Dr. Gaskell explains, is constructed out of a portion of the old 

 breathing apparatus. 



The fact that the pineal or median eye, existing and func- 

 tional in the arthropods of the Silurian age, is found present in 

 the larval lamprey, while in the adult it is superseded by 

 functional lateral eyes, the normal eyes of the vertebrate, is very 

 significant, as also the presence in the vertebrate eye only of 

 the characteristic IMullerian fibres which are not found in in- 

 vertebrates, but which Dr. Gaskell traces back to a process in 

 the alimentary canal. He asserts that the segmental excretory 

 organs of the arthropods have become respectively converted 

 into the pituitary gland, the tonsils, the thyroid, the thymus and 

 the glands of the lymphatic system ; the thyroid gland, in par- 

 ticular, he identifies very convincingly with the arthropoid 

 generative organs. 



The vertebrate heart he considers to have been evolved from 

 the junction of two venous channels in the lower (ventral) part 

 of the body, while the invertebrate dorsal heart has become 

 functionless and atrophied, and" exists as a mere mass of fat 

 above the spinal cord in the larval lamprey, disappearing alto- 

 gether in the higher vertebrates. 



The relationship between the respiratory muscles of the 

 arthropod and vertebrate appear to the layman obscure, and 

 Dr. Gaskell's explanations appear to require a professional 

 knowledge of anatomy and histology to be understood 

 thoroughly. The olfactory organs, however, are traced clearly 

 from their position in the scorpion to their subsequent situation 

 in the lamprey — an instance where the organ and its function 

 remain identical, but its position is changed; the converse of 



