ON THE SYSTEMATIC POSITION OF HESPERORNIS. 15 
While as to the small size of the brain, rather than make Hesperornis 
on this account a type ancestral to all existing birds two possible explana- 
tions suggest themselves. For bearing in mind the small size of the brain 
that Professor Marsx himself has demonstrated in so many of the great American 
fossil Mammalia, some of which are obviously very highly specialized and by no 
means primitive, we may judge either that immense structural specialization might 
and did go on without corresponding advance in cerebral development, or that the 
brain had actually a certain tendency to dwindle under the exceptional conditions 
of environment which conduced to the development of these huge extinct forms. 
And, lastly, we must not forget that the brain of very large animals is never quite 
as large in proportion, compared with smaller and closely allied forms. 
