No. 410.] THE MAMMALIAN SUBCLASSES. 125 
original separation of marsupials and placentals, because on a 
basis of arboreal habit we can explain the more important of 
the special characters which now distinguish the former. It is 
of interest to notice in this connection the opinion of Huxley : 
“I think it probable, from the character of the pes, that the 
primitive forms, whence the existing Marsupialia have been 
derived, were arboreal animals; and it is not difficult, I con- 
ceive, to see that, with such habits, it may have been highly 
advantageous to an animal to get rid of its young from the 
interior of its body at as early a period of development as 
possible, and to supply it with nourishment during the later 
periods through the lacteal glands, rather than through an 
imperfect form of placenta."! In other words, Huxley regarded 
arboreal habit as explaining two important characters of marsu- 
pials, namely the prehensile foot and premature birth. Now 
to these we may safely add the reduction of one of the denti- 
tions, because, as Leche ? has indicated, this is probably attrib- 
utable to the peculiar suckling conditions which are brought 
about by premature birth. We may also add the marsupium; 
Klaatsch (92) has shown that the perfect marsupium is a special 
character of the Marsupialia, and its *perfect development in 
this group may best be explained on a basis of arboreal 
habit. Naturally these suggestions cannot be regarded as 
at all final, but they point strongly to an association of the 
special characters of marsupials with arboreal habit. 
Passing by the results of Dollo, we find the idea of a pla- 
cental origin of marsupials also expressed by Wilson and 
Hill (97). These writers, in fact, originated this conception 
and produced what is perhaps the most significant evidence in 
its favor. 
The valuable joint contribution of Wilson and Hill (97) on 
the tooth development of Perameles, published in 1897, tended 
to confirm the idea, conceived by Huxley (80), but elaborated 
all representative of the Marsupialia in general In the Jurassic fauna of me 
northern hemisphere, and the Miocene fauna of diprotodonts of South America, 
we have two important groups for which an arboreal habit cannot as yet be 
proven. i 
1 Huxley ('80, p. 656). 
? Leche. Morph. Jahrb., Bd. xix, p. 525. 
