94 TJIE MAMMALIA. 



arrangement in the Monotrema. The young leave 

 the maternal womb in a very immature state. 

 the nourishment, the renewal of blood in the 

 uterus which, in the case of the Eutheria (the 

 high Mammals) is regulated by means of the 

 placenta for the advantage of the offspring, must, 

 owing to the absence of this foetal organ, be 

 accomplished at an early stage by mammary 

 glands. The further development of teats, in ac- 

 cordance with a beginning corresponding to the 

 marsupial pouch of the Echidna, has already been 

 mentioned. 



A fresh feature by which the Marsupials are 

 brought into direct connection with the higher 

 Mammals is their dentition. And, moreover, with 

 their dentition they extend back beyond the Mono- 

 trema to primary amphibian forms, and at the 

 same time the great variety of the forms prove, 

 in a most obvious way, that they have differen- 

 tiated from the simpler beginnings of their ancestors, 

 in so far as they were obliged to do so owing to 

 the generally more uniform surface of the earth. 

 One peculiarity affecting the whole group of Mar- 

 supials is this, that only one pair of teeth in either 

 jaw is changed during the lifetime. The succes- 

 sion of the teeth with the incoming of the one 



