172 E VOL UTION AND DISEA SE. 



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. Didelphys virginiana 

 has six on each side and one in the middle ; the crab- 

 eating opossum (Didelphys cancrivord) 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 



