36 THE INTERNATIONAL MONTHLY. 
descended ? Are the mammals monophyletic, i. e., were they 
all derived from a single stock ? or polyphyletic, i. e., derived 
from several stocks ? Unfortunately, these questions cannot yet 
receive a conclusive answer, for the evidence is yet insufficient, and, 
in particular, the origin of the class is still veiled in complete 
obscurity. Its history extends back to the Triassic period, the 
oldest of the three divisions of the Mesozoic era, but Triassic 
mammals are so excessively rare (only three or four specimens 
have been found) and so imperfectly preserved, that they give us 
very little information. In rocks older than the Triassic no fossils 
have been obtained which can be regarded as ancestral to mam- 
mals, but a highly important and suggestive group of reptiles is 
found in the Permian period, which just precedes the Triassic. 
This group, long entirely extinct, is known as the Theriodontia, 
and many of its members are surprisingly like mammals. Some 
of them, indeed, were referred to the mammals with little hesita- 
tion, when they were first discovered and imperfectly known. 
The theriodonts were long limbed, terrestrial forms, very widely 
separated from all other known reptiles, but in the character of 
the skull, limbs, and teeth approaching very near to the mammals. 
While no known theriodont can be regarded as ancestral to 
the Mammalia, some small, unspecialized forms which were so 
ancestral may well have existed and these hypothetical forms may 
be discovered at any time, and then the problem will be solved, in 
part at least. 
As just suggested, even the discovery of the hypothetical 
theriodonts may not clear up the problem of mammalian ances- 
try, because of the doubt as to the mode of their descent, whether 
from one group, or from more than one. If the latter was the 
mode (polphyletic origin) then it might prove that while some 
mammals were derived from theriodonts, others were descended 
from some very different group, which may not have been reptiles 
at all. The lowest of existing mammals, the monotremes, which 
include the duck-billed mole, and spiny ant-eater of Australia, 
reproduce by means of an egg, the young not being born alive, 
and these eggs greatly resemble those of reptiles. In very many 
