174 ANIMAL LIFE PAST AND PRESENT. 



hold good for all. Such is found in the circumstance 

 that in all cases the young are born in a very imperfect 

 and partially developed state, so that they resemble 

 little sacs or lumps; the young of the great lied 

 Kangaroo not being larger than our thumb. In this 

 exceedingly helpless and imperfect condition they are 

 transferred to the nipples of the female (which are 

 placed within the pouch when this is present), whereon 

 they hang without much signs of life, gradually grow- 

 ing as they are nourished by the milk, which is squirted 

 down their throats by a special muscle. 



In addition to this peculiarly early birth of the 

 young, and the very general presence of a pouch, the 

 Marsupials present several other curious structural 

 features, into the consideration of which it is not easy 

 to enter in a work like the present. It may, however, 

 be observed that they differ from other Mammals in 

 the circumstance that when milk-teeth (as the tem- 

 porary set of teeth which occur in the human infant 

 are termed) are developed, there is only one such tootli 

 in either side of each jaw, corresponding to the last 

 milk-tooth of the child (see Chapter XIV.). It appears, 

 indeed, that we have here the first commencement of 

 the development of a series of milk-teeth, which, in 

 their full stage, as seen in the child, are intended for 

 use during the period that the jaws are increasing in 

 size ; so that the permanent set may be of larger size 

 than would otherwise be possible. The absence of a 

 full series of milk-teeth at once indicates the low 

 organisation of the Marsupials ; and this is fully con- 



