THE EVOLUTION OF THE MAMMALIA. 37 
other respects the monotremes are so very different from all other 
mammals that several authorities believe that they must have 
descended from a different ancestral group of lower vertebrates, 
from that which gave rise to the other mammals. It should be 
noted, however, that the tendency of recent investigation does 
not favor the diphyletic origin of the mammals ; and if it should 
eventually prove that the monotremes have the same origin as 
the higher mammals, it will materially strengthen the opinion 
that the class has had its origin from a reptilian group. 
While the palaeontological evidence, fragmentary and incon- 
clusive as it is, seems to point to the reptilian origin of the 
Mammalia, the evidence of comparative anatomy and embryology 
points rather to another conclusion ; namely, that the ancestors 
of mammals are to be looked for among the Amphibia, a conclu- 
sion which seems to have been reached first by Huxley. The 
evidence for this view, while undoubtedly strong of its kind, can- 
not well be exhibited in an untechnical way. Palaeontologically 
it has no support, for no known group of fossil Amphibia at all 
fulfills the necessary conditions. 
The conclusion of the whole matter then is, that the question 
of the origin of the mammals is still an open one ; the class may 
have been derived from reptiles, from amphibians, or, perhaps, 
from some unknown class which was neither reptilian nor 
amphibian. Future discoveries must be awaited before the 
difficulties can be removed. 
Much the same uncertainty obtains as to the mutual relation- 
ships of the three great types of mammals, the monotremes, 
the marsupials, and the placentals. For a long time the preva- 
lent opinion was that of Huxley, that these three groups repre- 
sent three stages of mammalian descent, the monotremes having 
given rise to the marsupials, and the latter to the placentals. Of 
late, however, this view has given way to the opinion that the 
three groups are not ancestral, one to the other, but represent 
three diverging branches of one, or at most, two stocks. At all 
events, it is clear that the marsupials and placentals are very 
much more closely related to each other than either group is to 
