174 EVOLUTION AND DISEASE. 
As all cutaneous recesses exhibit a marked tendency 
to develop glands, the large size and abundance of the 
mammary glands within the marsupium of opossums 
receives an explanation. 
A comparison of the inguinal pouches in the sheep 
FIG. 92.—The marsupium of a Phalanger (Phalangista orientalis) 
opened to show the number of teats. 
with the marsupium of the phalangers, &c., is instructive, 
as it indicates in a significant manner the mode by which 
a specialized bag, or marsupium, may have originated, 
besides throwing light on the number of teats it con- 
tains. 
