HIGH SCHOOL ZOOLOGY. 143 



observed in familiar animals, still greater differences -characterise 

 two orders of the Mammalia which are almost confined to the 

 Australian Region — the Monotremes and the Marsupials. In 

 the first of these gi'oups eggs are laid, large in size, con- 

 taining much food-yolk, and surrounded by a shell which 

 has a certain amount of lime in its composition. After a 

 short period of incubation the young escape from the shell in 

 a very helpless condition, and are now dependent on the 

 milk which exudes from the cutaneous glands of the 

 mother. In one of the two genera which belongs to thiii 

 order, the milk-glands open into a pouch, big enough to receive 

 the head of the young, but in the other no such provision is 

 present. The Marsupials, the second of these orders, howevci-, 

 in which the young are also born in a very helpless condition 

 (the giant Kangaroo, as tall as a man, brings forth yoiing of 

 about the size of a newly-born rat), have a jjrovision for shelter- 

 ing them in the shape of the pouch, a fold of the skin on the 

 ventral surface of the abdomen, which supports and protects the 

 young while they are being fed by the milk-glands of the 

 mother, and to which they resort in danger even after they ai-e 

 able to run about. In this respect, then, we are able to i-eco^- 

 nize three gi-eat groups of Mainmalia, (1.) the oviparous forms; 

 (2.) those which bring fo)-th their young alive, but in such a 

 helpless condition that they must be carried for a long time in 

 the mother's pouch ; and (3.) the higher Mammals, in which the 

 development of the young advances to a much higher degree 

 within the body of the mother. Tliese gi-oups or sub-classes 

 have been called Prototheria, Metatheria, and Eutheria, i-e'spect- 

 ively, and while each of the first sub-classes contains only a 

 single order of Mammalia, the third embraces alL the remaining- 

 orders, and contains all the most familiar and cokspicuoos 

 quadrupeds. It is not to be supposed that this" primary sub- 

 division into sub-classes depends alone on the characters men- 

 tioned; there are certain other anatomical differences in the 



