THE EVIDENCE OF THE CENTRAL NERVOUS SVSTEM 17 
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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