172 EVOLUTION AND DISEASE. 
belly of the animal forms a pouch into which the 
young litter are received at their birth; where they 
have an easy and constant access to the teats ; in which 
they are transported from place to place ; where they 
are at liberty to run in and out ; and where they find a 
refuge from surprise and danger.” 
On the wall of this pouch formed by the mother’s 
belly we find nipples and milk glands; in kangaroos 
and phalangers four teats are usually present ; in opos- 
sums they are more numerous. D7zdelphys virginiana 
has six on each side and one in the middle; the crab- 
eating opossum (Didelphys cancrivora) has eleven teats, 
the odd one occupying the centre, whilst the remaining 
ten are disposed in a circle around it. 
Such luxuriance of nipples in so limited a space is 
unknown in mammals outside the order marsupialia, 
and in them is probably due to the protecting influence 
of the pouch, as the following evidence indicates :— 
Malkmus has recently drawn attention to some points 
of interest in the structure of the walls of the cutaneous 
recesses which exist in the inguinal region of sheep, or, 
as he terms them, rudimentary marsupia. On examining 
the inner aspect of a sheep’s flank we find it smeared 
with an unctuous material not unlike cerumen, or ear- 
wax. In the fold of each groin, close beside the dugs, a 
shallow recess is seen (fig. 91) usually containing a 
quantity of the same brown, unctuous material which 
besmears the flanks, and, as far as my observations 
extend, it is more abundant when the ewe has a lamb 
by its side. The walls of this recess are almost devoid of 
wool, but are beset with a number of large sebaceous 
