80 STRUCTURE OF THE VERTEBRATES 



As with most transitional groups, there are many characters 

 which, by comparison with the higher types, seem very ineffi- 

 cient; but when compared with lower forms their selective value 

 is apparent. This is true of the development of the marsupials. 

 They managed to survive the Age of Reptiles due to higher intel- 

 ligence and some care of the young; but when their more highly 

 developed descendants arose they all but disappeared. And the 

 majority of those which survived were in a region protected from 

 the higher mammals. There is a single exception to the above 

 description of development, but this will be mentioned in the 

 following chapter. 



Sub-class EuTHERiA. This group is also known as the Placen- 

 talia on account of the placenta, an embryological development 



Shrew (Insectivora) 



Fig. 29. Order Insectivora. The living representatives of the most primi- 

 tive placental order are externally specialized. The Order Primates rose 

 directly from the Insectivora. 



of the allantois which attaches the young to the wall of the 

 mother's uterus. Their distribution is world-wide except for 

 Australasia, where only the flying mammals were found until 

 others were naturalized by white men. Various types of placen- 

 tals were mentioned at the beginning of the chapter. 



The eggs are microscopic in size and almost lacking in yolk. 

 Growth depends upon food and oxygen being absorbed from the 

 blood stream of the mother, and the young are born in a fairly 

 well developed condition. The following grouping of orders is 

 simplified. Those which bear upon human evolution, or which 



