246 THE AMERICAN NATURALIST. [Vor. XXXV. 
hypothesis. A fuller discussion of the question will be pub- 
lished in the course of a few months. 
. The fact was noticed long ago by Cuvier, and has been com- 
mented upon by many zoólogists since, that the marsupials of 
Australia assume much the same róles in nature as are taken by 
placentals in other parts of the world, and that they show exten- 
sive resemblances to the latter in many features of their external 
organization. It is only within the last few years, however, in 
fact since the evolution of the placentals has come to be under- 
stood, that the exact significance of these resemblances has 
been at all apparent. Knowing that during the Tertiary period 
placental mammals have undergone a progressive development, 
or, in other words, an adaptive radiation, from diffuse or collect- 
ive types, by which not only their minor but also their ordinal 
characters have been established, the question now suggests 
itself, Have not the Australian marsupials undergone an entirely 
similar or, in other words, a parallel radiation ? 
The possibility of such a condition has already been sug- 
gested by Osborn (99a) who recognizes several mammalian 
radiations, including among them a marsupial radiation for 
Australia, and remarks as follows: * We mark the fact that 
the above radiations are all of ordinal rank, for the marsupial 
radii, although termed families, are adaptively equivalent to 
several placental orders.” 
Examining the composition of the group somewhat in detail, 
we find many indications of such a parallel radiation. Two 
classes of facts may be specially noticed in this connection. 
First, the Australian marsupials constitute a very homogeneous 
group; although differing widely in extremes, not only the 
various genera of a family but also the families themselves are 
connected by almost insensible gradations of structure. Second, 
the adaptive modifications of their teeth and feet are very 
similar to those of placental mammals either at the present 
day or during their progressive development in the Tertiary 
period, and they bear the same relation as do those of placen- 
tals to the general primitiveness of the animals in which they 
occur. 
The Australian marsupials, therefore, show no signs of a 
