118 THE AMERICAN NATURALIST. [VoL. XXXV. 
* En effet, personne ne soutiendra que zous ceux-ci ont passé 
par une phase arboricole. Et quant à ceux qui en sont là 
aujourd'hui, ce sont des types sporadiques, qui ont pris nais- 
sance indépendamment : la chose est évidente. 
* Si, donc, la nature du pied des Marsupiaux nous amène à ce 
résultat, que leurs ancétres immédiats étaient arboricoles, 
cette conclusion contribuera à appuyer ce que nous ont appris 
le placenta et la dentition du lait, notamment : 
«A savoir, que les Marsupiaux constituent un rameau latéral 
trés specialisé, et non le groupe générateur des Euthériens 
actuels." 
The essential element of Dollo’s theme is therefore an 
attempt to prove, by means of the structure of the foot, that 
the Marsupialia are primarily arboreal animals and that in 
respect to the possession of an arboreally modified foot they 
are too specialized to have constituted the source of the 
Placentalia. 
Dollo recognizes in the marsupial hind foot certain charac- 
ters of arboreal adaptation. These are (a) opposability of the 
hallux, (6) predominance of the fourth digit, with reduction 
and syndactylism of the second and third, and (c) recession of 
the claws. These characters represent successive stages in 
the development and perfection of a prehensile foot. Their 
purpose is essentially that of placing the great toe in opposi- 
tion to the outer digits, notably the fourth, in order to make 
the foot an efficient organ for grasping and holding fast to the 
limbs of trees. 
For convenience Dollo considers the marsupials in two series, 
— those which have the great toe well developed and func- 
tional, and those which have it reduced, or atrophied. This 
division is naturally one of habit ; the former are animals which 
have retained the arboreal habit, while the latter are animals 
which have abandoned it. In these the great toe, being no 
longer useful, has become reduced; 
To the first series belong the American opossums (Didel- 
phyida) and the Australian phalangers (Phalangerida). In 
the Didelphyide (Fig. 1, a), with the exception of the sub- 
genera Micoureus and Philander, opposability of the hallux is 
