No. 410.] THE MAMMALIAN SUBCLASSES. I 31 
former appearing at a slightly later period than thelatter. The 
Eutheria are depicted as giving rise, somewhere in the upper 
part of the Triassic, to the Marsupialia and Placentalia, which 
are represented in the Jurassic by the Triconodonta and Insec- 
tivora primitiva, respectively. The significant feature of this 
is the time relation which is implied by the recognition of 
marsupial and placental types among the Jurassic Mammalia! 
If we may argue from the probable sequence of events, it 
would appear that we have an important clue to the characters 
of the Jurassic forms, namely the single tooth change of Tri- 
conodon, providing that the latter is comparable to that of 
existing forms. For if the reduction of one dentition is, as 
Leche suggests, attributable to premature birth, and the latter 
is, as Huxley and Dollo suggest, in turn dependent upon arbo- 
real habit, then already in the Jurassic period the marsupials 
must have had their special characters well under way ; so that 
the common ancestors of marsupials and placentals must be 
placed, as Osborn has indicated, at some time prior to the 
Jurassic period. Naturally this gives us no clue to a placental 
or non-placental origin of marsupials, but the distinction, if 
valid, is an important one, because it avoids the confusion which 
would result from an attempt to identify and characterize the 
common ancestors among the more complex faunas of later 
periods. But more especially it minimizes the importance 
of the arboreal characters of the marsupials of later periods 
and renders necessary a knowledge of the foot structure of 
Jurassic forms before decisive proof of arboreal ancestry may 
be obtained. 
2. The Question of a Polyphyletic Origin of the Mammalia. 
As indicated above, the view of Dollo, Wilson, and Hill 
opposes that of Huxley merely in reversing the relationships 
of marsupials and placentals while retaining the idea of a gen- 
eral continuity of the Mammalia. The views of other writers, 
on the other hand, substitute for continuity an independent 
origin and development of the main mammalian stems. 
'91, p. 115). 
! Proposed in 1888 ; cf opposite opinion of Flower and Lydekker (91, p. 115) 
