116 GEO. S. HUNTINGTON 
requirements of their modern descendants, that in the majority 
of these a very considerable reduction of the respiratory area 
was made possible and actually effected in the course of evolu- 
tion, leaving the relatively small number of extant bilaterally 
eparterial forms as representatives of a preceding archeal phy- 
letic stage. The palaeontological evidence bearing on the origin 
of the mammalia declares the contrary to have been the case and 
characterizes the earliest mammalian or promammalian types as 
exceedingly small animals, about the size of a shrew, with ter- 
restrial, probably arboreal, habitat. Mandibular remnants and 
teeth form practically the only evidence of their presence dur- 
ing the entire geological period of enormous length extending 
from the mesozoic to the upper cretaceous. 
Palaeozoologists ascribe this long continued suppression of 
the mammalian stem to unfavorable environment, chiefly due to 
the domination of the mesozoic reptiles. In the upper limits of 
the cretaceous period many of the large reptilian phyla appear 
to have been suddenly destroyed, and in consequence the mamma- 
lia began a very rapid course of evolutionary development and 
advancement. The earliest well known faunae of placental 
mammalia date from the beginning of the Tertiary period (basal 
and lower eocene). In the insectivora, primates, carnivora, - 
condylarthra, amblypoda, edentata, the most primitive forms, 
least- specialized in dentition and limbs, are always small ani- 
mals of terrestial habitat. The most primitive insectivores 
were even minute and shrew-like in form, the most primitive 
carnivores and archaic ungulates about as big as a large opos- 
sum. Gregory and Mathew hold that the immediately ances- 
tors of the early Eocene faunae were arboreal types more or 
less resembling the modern opossum in appearance and habits. 
The palaeontological evidence as to the origin of the Marsu- 
palia is slender, but supports the opinions of Huxley, Dallo, and 
Bensley, that both the Polyprodont and Diprodont types have 
been derived from small arboreal forms resembling the Didel- 
phyidae in most characters. The marsupalia as a group appear 
to represent endforms of a mesozoic pre-placental stock of low 
brain structure and more or less arboreal habitat. 
