CHAPTER XAVI 
Crass MAMMALIA 
1, PROTOTHERIA; 2. METATHERIA; 3. EUTHERIA 
Birps and Mammals have evolved along very different 
lines, Birds possessing the air and Mammals the earth, and 
it is difficult to say that either class is the higher. But 
apart from the fact, which prejudices us, that man himself 
is zoologically included among Mammals, this class is 
superior to Birds in two ways—in brain development, and 
in the relation between mother and offspring. In most 
Mammals there is a prolonged organic connection between 
the mother and the unborn young, which may have been, 
as Robert Chambers suggested, one of the conditions of 
progress. It is also characteristic of Mammals that the 
young are nourished after birth by their mother’s milk, and 
it has been suggested that the usually prolonged infancy 
was one of the factors in the evolution of the humaner 
feelings. It is certain at least that the carefulness and 
sacrifice of the mothers has been one factor in the survival 
and success of Mammals, and we may find in the term 
Mammalia, which Linneeus first applied to the class, a hint 
of the idea that in the evolution of this class the mothers 
led the way. 
GENERAL SURVEY OF MAMMALS 
There are three grades of Mammalian evolution :— 
A. The duckmole (Ornithorhynchus) and the spiny 
ant-eaters (Zchidna and Proechidna) difler very markedly 
from all other Mammals. The young are hatched outside 
