416 Life and Lnmortatity. 
gigantic than those of their nearest living relatives. More- 
over, these ancient forms of life represent what are called 
comprehensive types, or types that possess characters in 
combination such as are nowadays found separately devel- 
oped in different groups of animals. Such permanent reten- 
tion of embryonic characters and comprchensiveness of 
structural type are signs of what zodlogists consider to be 
comparatively low grades of organization, and their preva- 
lence in the earlier forms of animals is a very astonishing 
phenomenon, though they are none the less perfectly organ- 
ized so far as their peculiar type is concerned. As we ascend 
the geological scale, these features will be found to gradu- 
ally disappear, higher and even higher forms will be intro- 
duced, and specialization of type take the place of the former 
comprehensiveness. ‘That there has been in the past a gen- 
eral progression of organic types, and that the appearance of 
the lower forms of life has in the main preceded that of the 
higher forms in point of time, is a widely-accepted generaliza- 
tion of palzontology. 
Now that it has been seen that there has been a gradual 
progression and development of animal types all through 
the ages up to the era of man, the question naturally occurs 
whether or not the changes are still going on which will 
result in a higher development. Man coexisted in Western 
Europe with several remarkable mammals in the later por- 
tion of the Post-Pliocene Period. While we do not know 
the causes which led to the extinction of the mammoth, 
woolly rhinoceros, cave-lion and others, yet we do know that 
scarcely any mammalian species have become extinct during 
the historical period. The species with which man coexisted 
are such that presumably required a very different climate to 
that now prevailing in Western Europe. Some of the depos- 
its in which man’s remains have been found in association 
with the bones of extinct mammals incontestably show that 
great changes in the physiography and surface-configuration 
of the country had taken place since the period of their 
