CHAP. XIII 



BACKBONED ANIMALS 



291 



There are many reasons for thinking that the Rat it re do 

 not form a natural group. *\ 



The most reptilian, least bird-like of birds is the oldest 

 fossil of all, plaeed in a 

 sub-class by itself, the Ar- 

 chceopteryx (lit. ancient 

 bird) from strata of Jurassic 



age. 



9. Mammalia. On a quite 

 different line of evolution 

 from birds, and usually 

 excelling them in brains, 

 mammals are typically ter- 

 restrial, four-footed, and 

 hairy. Bats and whales, 



/ 



seals and sea-cows, are ob- 

 viously exceptional. The 

 brain of mammals is more 

 highly developed than that 

 of other animals, and in the 

 great majority there is a 

 prolonged (placenta!) con- 

 nection between the unborn 

 young and the mother. In 

 all cases the mothers feed 

 the tender young with 

 milk. 



In the class there are three grades :- 



(1) In the Duckmole (Orniihorhynchus\ the Porcupine 

 Ant-eater (Echidna), and another genus Proecliidna, the 

 females lay eggs. In many other ways these exclusively 

 Australasian mammals are primitive, exhibiting affinities 

 with reptiles. 



(2) In the Marsupials, which, with the exception of the 

 American Opossums and Selvas, are also Australasian, 

 the young are born at a very tender age, as it w r ere, 

 prematurely. Only in one genus (Perameles) is there a 

 true placenta as in the higher mammals. In the great 

 majority, the mothers put the newborn young into an 

 external pouch, where they are fed and sheltered till able 



FIG. 95. RESTORATION OF THE EX- 

 TINCT MOA (Dinornis ingcns), AND 



ALONGSIDE OF IT THE LITTLE 



KIWI (Apleryx manlelli). 



(From Chambers's Encyclop. ; after 

 F. v. Hochstetter.) 



