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 



