THE PHYLA HEMICHORDATA AND CHORDATA 



Thus, mammals of small size were present as an insignificant fraction of the 

 population during the millions of years that witnessed the rise and dominance 

 of dinosaurs and other ruling reptiles and the rise of birds. The fossil 

 record is incomplete; but if there had been many larger species of mammals, 

 some would certainly have been preserved as fossils and found before this. 

 Only toward the close of the Mesozoic, which marks the end of the Age of 

 Reptiles, did the diversification of the mammals begin. In the Age of 

 Mammals, when this class of warm-blooded vertebrates became dominant 

 upon the land, they seem to have replaced reptiles by an expansion into 

 territory that was being relinquished. The mammals were the more efficient 

 of the two; they had warm blood, which enabled them to range at will, and 

 greater possibilities of locomotion, to say nothing of wits. What seems to 

 have happened is not that the more efficient type drove out the less efficient 

 but that the reptiles declined for some unknown reason, and so the land again 

 became free for new occupants. The mammals were at hand and became the 

 dominant land forms of the Tertiary, or Age of Mammals, as the early 

 amphibians and reptiles had become dominant upon the older land surfaces. 

 The early mammals were small insect eaters, but diversification into the 

 principal mammalian types was rapid, and with increase in size of the in- 

 dividuals in many lines the great mammalian fauna of the Tertiary came into 

 being. Like the fishes, amphibians, and reptiles, the mammals had their day; 

 they were a waning race even before many recent forms were confronted with 

 ultimate extinction through the activities of Homo sapiens. Representative 

 types of existing mammals are shown in Figures 18.30-18.39. 



Along with that of other mammals, the human line of descent begins with 

 the insect eaters of the trees at the close of the Age of Reptiles. Surviving 

 oflTshoots that mark the path of this evolution are the existing lemurs, tarsiers, 

 monkeys, and great apes; this descent is confirmed by what is known from 

 fossils. Only in the late Tertiary or earliest Pleistocene, it seems, did 

 our ancestors descend from the trees; binocular vision and important quali- 

 ties of hand and brain, along with the beginnings of an upright posture, were 

 established before man's forebears came to earth. The later phases of human 

 evolution are outlined in Chapter 20. Speaking of our ancestors in the early 

 Tertiary, W. D. Matthew describes their appearance as intermediate between 

 that of a lemur and a mongoose. They were animals "rather catholic in their 

 tastes, living among and partly in the trees, with a sharp nose, bright eyes 

 and a shrewd little brain behind them, looking out, if you will, from a perch 

 among the branches, upon a world that was to be singularly kind to them and 

 their descendants." 



Summary 



The phylum Chordata includes certain invertebrate animals along with 

 the familiar vertebrates. The species representing the lowly members of the 



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