CHAPTER VIII 



GREAT STEPS IN ORGANIC EVOLUTION 



THE ASCENT OF LIFE, OR THE FAUNA AND FLORA OF 

 SUCCESSIVE GEOLOGICAL AGES 



In the ecological chapter an outline was given of the spatial distribu- 

 tion of organisms in various regions of land and sea; we turn now to 

 the distribution in time. Each geological age has had its character- 



TERTIARY 

 on 



CACNOZOlC 



SeCONDAR-v 



OK 

 HtSOZOIC 



PALACOrOIC 



RISE OF MAMMALS 



-FIRST BIRO rOSSILS 



, OOLOCN AOE OF 

 REPTILES 



FIRST TRACES OF 

 MAMMALS 



FIRST AMPMlBIAN 

 SKELETONS 



.« « DEVONIAN ' *' ' 



c c 



C C < C I I c 



< ' 'OROOVICIAN < •■ 

 >.<<■'■ i. I itt tn«.<. 



C(.(.(.(CC l.<.(l<. 



t.tcit<».<.t«.».« 

 .tilt tctci 



FIRST FISHES 



MOKE invERTCBRATCS 



MANY IMWtRTeerUfTES 



I I M M l_ I M I I 



i I I \ 1 I i I i i i i 

 I I ' I I ' ' i I i > I 

 ; I r ARCHACAM i 'li 

 \ll '. '. '.Ill it; 

 1 I I I 1 '. ^ 1 l '. •.) 



' ' ' '. I I I ! I I l! 



I r I* r I r 1 « I « i' 



Fig. 137. 



Diagram showing the Successive Geological Formations composing the 

 Crust of the Earth. The later periods are perforce exaggerated in thickness 

 in proportion to the much longer earlier periods. 



istic plants and animals, and the succession has continued for about 

 a thousand million years — more or less — on the latest calculation 

 based on the combined estimates of geologists and radiologists. 

 The largest fact disclosed by the study of geological distribution 



VOL. II G 



