THE LINEAGE OF MAN 



as we pass from the oldest amphibians to the oldest reptiles ; 

 thence to the theromorph series of reptiles, which became 

 more and more mammal-like; thence to the oldest mammals, 

 onward to the most primitive placental mammals of the 

 closing age of the dinosaurs; thence to the tree-shrews, 

 lemuroids, monkeys and apes that grow steadily more like 

 man; finally to man himself in the last few million years 

 of the billion-year history of life. 



Origin of the Reptiles 



In their individual development or embryology the 

 amphibians, like the fishes, went through a water-living stage 

 of development, in which they had functional gills; but the 

 earliest reptiles succeeded in laying their eggs and rearing 

 their young wholly upon land and in this way were able not 

 only to invade the drier uplands and many parts of the earth 

 where water was scarce, but to avoid the intensive competi- 

 tive warfare for living food that must have raged in the 

 swamps of the coal forests. 



The earliest reptiles were still so much like their con- 

 temporary relatives, the amphibians, in most of their skeletal 

 characters that some of them were on or relatively near the 

 borderland between the two classes. Such a form is 

 Seymoufia, from the Permian of Texas, a fine skeleton of 

 which is mounted in the University of Chicago. The pattern 

 of its skull bones, as seen from above and from the side, 

 conforms closely to the prilnitive amphibian type and is an 

 almost ideal archetype of every later skull, including that 

 of mammals and of man himself. The same is true of the 

 underside of the skull, including the arrangement of the 

 numerous elements of the upper jaws, palate, and base of 

 the cranium. It may be said of Sey^nouria, as of many other 

 generalized forms, that, if we had not discovered them, we 

 could have predicted their existence, so closely do they 



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