HOW THE MAMMALS DEVELOPED 209 
mother, covering her breasts. This forms a suitable 
resting place into which these young are conveyed as 
soon as they are born and from which they do not 
emerge for many days. The little creature instantly 
fastens upon the nipple of the mother, keeping its 
mouth constantly in this position. At intervals the 
muscles of the breast force the milk into the mouth 
of the young, which is still too undeveloped to suck 
for itself. As it gets older the little opossum or kan- 
garoo emerges from the pouch in the pleasanter part 
of the day and in the absence of danger. It returns 
to the mother’s pocket as soon as it becomes cold or 
a cry from its parent warns it of its defenseless po- 
sition. 
These creatures are the lowliest of the class upon 
the earth. The great majority of all mammals have 
elaborated a far finer plan, in which the young are 
retained within the body of the parent until they are 
quite able to stand the air. The length of this time 
varies in different mammals from a few weeks to 
more than a year. The egg must be fertilized before 
it leaves the body of the parent. If it should fail in 
this it simply passes out and is wasted. If the fer- 
tilizing cell reaches the egg before it has progressed 
far down the tube it begins its development. The 
embryo forms for itself the sort of head and tail 
and gill slits which would have served its fish or its 
