XIII 
THE GEOLOGICAL EVIDENCES OF EVOLUTION 
391 
veloped types—exactly such a change as we may expect to 
find if the evolution theory be the true one. Many other 
illustrations of a similar change could be given, but the 
animal groups in which they occur being less familiar, the 
details would be less interesting, and perhaps hardly intel¬ 
ligible. There is, however, one very remarkable proof of 
development that must be briefly noticed—that afforded by 
the steady increase in the size of the brain. This may lie 
best stated in the words of Professor Marsh :— 
“ The real progress of mammalian life in America, from 
the beginning of the Tertiary to the present, is well illus¬ 
trated by the brain-growth, in which we have the key to 
many other changes. The earliest known Tertiary mammals 
all had very small brains, and in some forms this organ was 
proportionally less than in certain reptiles. There was a 
gradual increase in the size of the brain during this period, 
and it is interesting to find that this growth was mainly 
confined to the cerebral hemispheres, or higher portion of the 
brain. In most groups of mammals the brain has gradually 
become more convoluted, and thus increased in quality as 
well as quantity. In some also the cerebellum and olfactory 
lobes, the lower parts of the brain, have even diminished in 
size. In the long struggle for existence during Tertiary time 
the big brains won, then as now; and the increasing power 
thus gained rendered useless many structures inherited from 
primitive ancestors, but no longer adapted to new conditions.” 
This remarkable proof of development in the organ of 
the mental faculties, forms a fitting climax to the evidence 
already adduced of the progressive evolution of the general 
structure of the body, as illustrated by the bony skeleton. 
We now pass on to another class of. facts equally suggestive 
of evolution. 
The Local Relations of Fossil and Living Animals. 
If all existing animals have been produced from ancestral 
forms—-mostly extinct — under the law of variation and natural 
selection, we may expect to find in most cases a close rela¬ 
tion between the living forms of each country and those which 
inhabited it in the immediately preceding epoch. But if 
species have originated in some quite different way, either by 
