THE EVIDENCE OF THE CENTRAL NERVOUS SYSTEM 21 
struggle for existence is essentially between the members of that 
race, and not between them and any inferior race. 
The effect of such conditions is, as Darwin has pointed out, to 
cause great variation in that group; in consequence of that variation 
and that dominance the evolution of the next higher group is brought 
about from some member of the dominant group. Thus the present 
age is the outcome of the Tertiary period, a time when giant mammals 
roamed the earth and left as their successors the mammals of the 
present day; a time of dominance of quadruped mammals; a time 
of which the period of maximum development is long past, and we 
now see how the dominance of the biped mammal, man, is accom- 
panied by the rapid diminution and approaching extermination of 
the larger mammals. No question can possibly arise as to the im- 
mediate ancestor of the biped mammal; he undoubtedly arose from 
one of the dominant quadrupedal mammals. 
Passing along to the next evidence of the rocks, we find an age of 
reptiles in the Mesozoic period. Here, again, the number and 
variety is most striking; here, again, the size is enormous in com- 
parison with that of the present-day members of the group. This 
was the dominant race at the time when the birds and mammals 
first appeared on the earth, and anatomists recognize in these extinct 
reptilian forms two types; the one bird-like, the other more mamma- 
lian in character. From some members of the former group birds 
are supposed to have been evolved, and mammals from members of 
the other group. There is no question of their origin directly from 
lower fish-like forms; the time of their appearance on the earth, 
their structure, all point irresistibly to the same conclusion as we 
have arrived at from the consideration of the origin of the biped 
from the quadruped mammal, viz. that birds and mammals arose, in 
consequence of the struggle for existence, from some members of the 
reptilian race which at that time was the dominant one on earth. 
Passing down the geological record, we find that when the reptiles 
first appear in the Carboniferous age there is abundant evidence of 
the existence of numbers of amphibian forms. At this time the 
giant Labyrinthodonts flourished. Here among the swamps and 
marshes of the coal-period the prevalent vertebrate was amphibian 
in structure. Their variety and number were very great, and at that 
period they attained their greatest size. Here, again, from the 
geological record we draw the same conclusion as before, that the 
