THE EVIDENCE OE THE CENTRAL NERVOUS SYSTEM 1 7 



either by progress upwards to a superior form, or by degeneration to 

 a lower type of animal. The principle of degeneration as a factor 

 in the formation of groups of animals, which are thereby enabled 

 to survive, is nowadays universally admitted. The most striking 

 example is to be found in the widely distributed group of Tunicata, 

 which live, in numbers of instances, a sedentary life upon the rocks, 

 have the appearance of very low forms of animal life, propagate 

 by budding, have lost all the characteristics of higher forms, and 

 yet are considered to be derived from an original vertebrate stock. 

 Such degenerate forms remain degenerate, and are never known to 

 regenerate and again to reach the higher stage of evolution from 

 which they arose. Such forms are of considerable interest, but 

 cannot help, except negatively, to decide what factor is especially 

 important for upward progress. 



At the head of the animal race at the present day stands man, 

 and in mankind itself some races are recognized as higher than others. 

 Such recognition is given essentially on account of their greater 

 brain-power, and without doubt the great characteristic which puts 

 man at the head is the development of his central nervous system, 

 especially of the region of the brain. Not only is this point most 

 manifest in distinguishing man from the lower animals, but it applies 

 to the latter as well. By the amount of convolution of the brain, 

 the amount of grey matter in the cerebral hemispheres, the enlarge- 

 ment and increasing complexity of the higher parts of the central 

 nervous system, the anthropoid apes are differentiated from the lower 

 forms, and the higher mammals from the lower. In the recent work 

 of Elliot Smith, and of Edinger, most conclusive proof is given that 

 the upward progress in the vertebrate phylum is correlated with the 

 increase of brain-power, and the latter writer shows how steady and 

 remarkable is the increase in substance and in complexity of the 

 brain-region as we pass from the fishes, through the amphibians and 

 reptiles, to the birds and mammals. 



The study of the forms which lived on the earth in past ages con- 

 firms and emphasizes this conclusion, for it is most striking to see 

 how small is the cranium among the gigantic Dinosaurs ; how in the 

 great reptilian age the denizens of the earth were far inferior in brain- 

 power to the lords of creation in after-times. 



What applies to the vertebrate phylum applies also to the inver- 

 tebrate groups. Here also an upward progress is recognized as we 



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