180 PKOF. JOHN CLELAND, M.D., ETC., ON 



mal? jS^o such denionstratinn has been seriously attempted. 

 On the other hand the determinate nature of Evolution is in 

 my humble opinion quite demonstrable. To that extent 

 Oken was in the right ; but the conception which he termed 

 Mathesis is better laid hold of, though still incompletely, by 

 using the word Design. 



It is such considerations as these which invest the classi- 

 fication of the Vertebrata Avith general interest. Let us 

 examine dispassionately how vertebrate animals differ from 

 others, before we proceed to make a rapid survey of the 

 characters of its great divisions. The fundamental charac- 

 teristic is to be found not really in the vertebral column so 

 much as in that structure which the column protects and 

 supports, namely, the great nervous centre, the cerebro- 

 spinal axis. This centre, though divisible into brain and 

 vspinal cord, forms a single continuous structure, beginning in 

 the region of the head and extending away from it originally 

 in the embryo, placed at first superficially, but soon converted 

 into a continuous tube with skeletal surroundings. It at no 

 period presents the appearance of a gangliated chain, though 

 it gives off the nerves in pairs. Its position is dorsal, while 

 the heart is on the ventral side of the alimentary canal. The 

 whole vertebrate body presents a segmented arrangement, 

 that is to say, a serial chain of repeated parts, a phenomenon 

 no doubt pervading many of the Invertebrates; but that 

 which distinctly characterises the vertebrate segmentation is 

 that it is one in which the outgrowths of the cerebro-spinal 

 axis take prominent part, and is a segmentation of the 

 specially animal sphere, not of the visceral systems, although 

 these exhibit a certain serial repetition of a more or less 

 independent kind. 



It is now twenty-two years since, in a popular text-book on 

 the structure and functions of the human body, I referred to 

 the relationship which had been pointed out between Verte- 

 brata and Tunicata ; stating that the constant ganglion of the 

 latter might fairly be considered as homologous with the 

 anterior or preoesophageal ganglion of Articulata, and that it 

 was probable that the cerebro-spinal axis of vertebrates was 

 " a highly developed structure corresponding with that one 

 ganglion." At that time the idea which had been put forward 

 and greatly favoured was that from the Tunicata ascent took 

 place through x\mphioxus to the Vertebrates proper ; the part 

 ^jlayed by the notochord in that supposed evolution being 

 much insisted on. It has ^ince occurred very justly to the 



