432 



ZOOLOGY . 



Discovery of 

 the land 

 by man's* 

 ancestors 



Egg-laying 

 mammals 



The amphibian, the next step in the path of evolution 

 toward man, unquestionably arose from a relatively 

 primitive type of fish. 



6. With the appearance of amphibians came the dis- 

 covery of the land by vertebrates, as we have already 

 indicated. Next came the reptiles, capable of repro- 

 ducing without recourse to water. From these arose the 

 mammals, but not from anything like the modern rep- 

 tile. As in the case of the fishes, the higher reptiles, 

 with their single occipital condyle and other peculiari- 

 ties, have gone off on a path which cannot possibly lead 

 to anything mammalian. It is only by reference to 

 very ancient fossil forms that we can get any accurate 

 clew to the course of events. This we seem to find in 

 the cynodont or dog-toothed reptiles of the South 

 African Mesozoic, animals which possessed paired 

 occipital condyles, and the teeth differentiated into in- 

 cisors, canines, premolars, and molars. 



7. The first true mammals, or Prototheria, were 

 warm-blooded, hairy, egg-laying creatures. The mod- 

 ern Australian duckbill (Ornithorhynchus) is a special- 

 ized member of this group. The principal food of these 

 animals was probably insects, and it is perhaps a fact 

 that the development of the mammalian type was 

 largely aided and made possible by the increasing de- 

 velopment and variety of insect life. Still in the Meso- 

 zoic age, primitive marsupial mammals arose, now 

 viviparous, but producing the young in a very under- 

 developed condition, so that they had to be nourished 

 in the maternal pouch. Such animals, represented 

 today by the opossums, were probably also insect 

 feeders, like the South American Marmosa. They were 

 also probably arboreal, living in trees ; and in accord- 

 ance with these habits certain changes took place in the 



