The Consequences of Evolution - 569 



until the middle Mesozoic strata, and only 

 in the upper Mesozoic strata does one begin 

 to find flowering plants and modern insects. 

 The close of the Mesozoic era also marks the 

 extinction of most of the numerous ancient 

 reptilian hordes that had dominated the land 

 and air for a hundred million years. Hardly 

 a single fossil trace of the many kinds of 

 dinosaurs, pterosaurs, and other primitive 

 types of reptiles are to be found in any of 

 the Cenozoic strata. 



The Cenozoic era, in which we live, is 

 characterized by a very abundant fossil rec- 

 ord; but to consider the details of this record 

 would carry us far into the field of paleon- 

 tology. Very briefly it can be said, however, 

 that the Cenozoic era is the age of mammals 

 and angiosperms. 



By the beginning of the Tertiary period 

 quite a variety of archaic mammals had 

 usurped the place of the dinosaurs. These 

 mammals displayed a very primitive struc- 

 ture. Their teeth, unlike the differentiated 

 cutting (incisor), tearing (canine), and grind- 

 ing (molar) teeth of modern mammals, were 

 all essentially the same — like typical reptilian 

 teeth. Moreover, the archaic mammals 



Fig. 29-16. A primitive Jurassic bird, Archaeopferyx. 

 Note the teeth, the elongate tail, and the clawed 

 wings. (Courtesy of the American Museum of Natural 

 History, New York.) 



walked, not on their toes, but on the soles 

 of their feet; they had five (the full primi- 

 tive number) of digits on both fore and hind 

 feet; and their cranial cavities were small 

 compared to those of most modern mammals. 

 In fact, very few of the archaic mammals 

 survived until the end of the Tertiary period. 

 Gradually they were displaced by the an- 

 cestors of the modern types, which had 

 begun to appear simultaneously in early Ter- 

 tiary times. 



By the middle of the Tertiary period most 

 of the modern orders of mammals (p. 673) 

 had become numerous and well established. 

 Here we find the richly documented evolu- 

 tionary stages of the Ungulata (horses, camels, 

 elephants, cattle, swine, etc.); the Carniv- 

 ora (tigers, lions, cats, dogs, etc.); Rodentia 

 (squirrels, rabbits, rats, mice, etc.); Primates 

 (monkeys, apes, lemurs, etc.); and so forth. 

 In fact, the peak of the ascendancy of the 

 mammals (excluding man) was reached be- 

 fore the close of the Tertiary period. In the 

 latter part of this period, there were great 

 herds of mastodons (Fig. 29-17); but these 

 elephantine creatures became extinct before 

 the onset of the recent epoch. 



The Tertiary period also records the rise 

 of the angiosperms to ascendancy in the plant 

 kingdom. Particularly important was the 

 spread of the grasses, which provided food 

 for the herbivorous mammals; and the exten- 

 sion of the deciduous forests, which afforded 

 protection for a wide variety of mammalian 

 types. 



The first manlike fossils began to appear 

 quite late in the Tertiary (Fig. 29-4); but un- 

 deniable evidence of the genus Homo is not 

 found until early in the Quaternary period. 

 The fossil record of the descent of man is 

 not so complete as that of many other mam- 

 mals (for example, the horse, camel, elephant, 

 etc.), due probably to the fact that the forest 

 habitat and mode of life among the early 

 primates were not conducive to fossilization. 

 However, the evidence is clear enough to 

 show that man owes his origin to apelike 

 creatures (Fig. 29-18), which in turn were de- 



