258 THE AMERICAN NATURALIST. [Vor. XXXV. 
A DIDELPHYID ORIGIN IN THE LIGHT OF GEOGRAPHICAL 
DISTRIBUTION. 
In discussing the question of the origin of the Australian 
marsupials it is apparent that, apart from the evidence deriv- 
able from the organization of existing types, any opinions 
which are expressed on the subject must take into consid- 
eration the present and past geographical distribution of 
marsupials, the manner in which the ancestral forms gained 
access to the Australian region, and the time at which their 
entry was effected. 
Within the last twenty-five years several attempts have 
been made to connect the Australian marsupials with the 
extinct types of different horizons in other countries, and to 
explain in this way the conditions of their origin. Thus, in 
1876, Wallace suggested a possible relation to the Jurassic 
forms of the northern hemisphere. Concerning this he 
speaks as follows: * As, however, no other form but that of 
the Didelphyidz occurs there (in Europe) during the Tertiary 
period, we must suppose that it was at a far more remote 
epoch that the ancestral forms of all the other marsupials 
entered Australia; and the curious little mammals of the 
Oólite and Trias offer valuable indications as to the time when 
this really took place"? . . . «It was probably far back in 
the Secondary period that some portion of the Australian region 
was in actual connection with the northern continent; and 
became stocked with the ancestral forms of marsupials.’ 3 
In 1882 Cope, in commenting upon the ancestry of Thyla- 
coleo, placed this form in the family Plagiaulacide of the 
northern hemisphere, and regarded the latter as related to the 
Australian Macropodidze through a hypothetical ancestor Trito- 
modon. There is, however, no valid evidence in favor of this 
view. 
! Haeckel has recently endeavored to express such a relation phylogenetically 
by deriving all of the mo Pr a, 
3 Ibid. (vol. i, p. 465) 
. 
