236 TRANSACTIONS OF THE [JUNE 4, 
for progressive evolution. The terms inertia potential are new 
in biography, but they seem to express most perfectly the differ- 
ences between certain great groups of mammals. As regards 
their progress in a similar period of time, we may compare cer- 
tain groups as follows: 
AAs, B. C. 
Marsupials and 
Monotremes. Mesoplacentals. Cenoplacentals. 
Inertia. Maximum. Medium. Minimum. 
Potential. Minimum. Medium. Maximum. 
The inertia is seen in the inability to shake off the reptilian 
inheritance and assume the typically mammalian stamp. Wher- 
ever they have come into competition, the Cenoplacentals have 
driven out the Mesoplacentals, just as the Placentals will in time 
supersede the Marsupials of Australia. 
The exception to this is, I believe, seen in two great groups 
still existing, but which are universally regarded as extremely 
ancient. 
These are the Insectivora and Lemuroidea, which may prove 
to be persistent Mesozoic types. The physiological division of 
the Placental orders would, according to this hypothesis, be as 
follows: 
EUTHERIA. 
Prototheria. Mesoplacentalia. Cenoplacentalia. 
Amblypoda. Dinocerata...Proboscidea. 
Monotremata. Coryphodontia. 
Condylarthiayc.sve baat acter Diplarthra, Artio- 
dactyla and Pe- 
rissodactyla. 
Creodonta. aoe hion ste ee = alee Carnivora. 
Multitubercu- Tillodontia. ...... «sy 62 hee EOCENE. 
culata. Cheiroptera. 
Insectivora. 
Lemuroidea. 
Anthropoidea. 
Incertz sedis; Edentata, Sirenia, Cetacea. 
The order Bunotheria of Cope, inculding the Creodonta, 
Mesodonta, Insectivoro, Tillodonta and Taeniodonta, was pro- 
