290 THE HISTORY OF CREATION. 
ance, be admitted to be the descendants of a single common 
primary form, namely, of the Protamnion. 
SIXTEENTH STAGE: Primary Mammals (Promammalia). 
We now find ourselves more at home with our ancestors. 
From the sixteenth up to the twenty-second stage they 
all belong to the large and well known class of Mammals, 
the confines of which we ourselves have as yet not 
transgressed. The common, long since extinct and unknown 
primary forms of all Mammalia, which we have named 
Promammalia, were at all events, of all still living animals, 
of the class most closely related to the Beaked animals, or 
Ornithostoma (Ornithorhynchus, Echidna, p. 233). They 
differed from the latter, however, by the teeth present 
in their jaws. The formation of the beak in the Beaked 
animals of the present day must be looked upon as an 
adaptive characteristic which developed at a later period. 
The Promammalia arose out of the Protamnia (probably 
only at the beginning of the secondary period, namely, in 
the Trias) by various advances in their internal organis- 
ation, as also by the transformation of the epidermal scales 
into hairs, and by the formation of a mammary gland 
which furnished milk for the nourishment of the young 
ones. The certain proof that the Promammalia—inasmuch ~ 
as they are the common primary forms of all Mammals— 
also belong to our ancestors, lies in the comparative 
anatomy and the ontogeny of Mammalia and Man. 
SEVENTEENTH STAGE : Pouched Animals (Marsupialia). 
The three sub-classes of Mammalia—as we have already 
seen—stand in such a relation to one another that the 
