360 



ANIMAL BIOLOGY 



mony to the order of appearance on the Earth of the different 

 types of animals and plants. (Fig. 232.) 



The geological time-table (page 359) summarizes the panoramic 

 succession of life as it is seen by the paleontologist. It is of little 

 value to discuss the absolute duration of geologic time, because 

 the estimates vary so greatly, though there are fairly reliable data 

 in regard to the relative length of the various eras. Perhaps the 

 conservative estimate of two billion years — much more than half 



FISHES 



AMPHIBIANS REPTILES 



BIRDS 



MAMMALS 



Fig. 232. — Chart showing the origin and degree of development of the. 

 chief groups of Vertebrates, correlated with the geological time-table. (From 

 Lindsey, after Osborn.) 



of which was before the Cambrian period — will serve to spell the 

 Earth's unfathomable past and to afford some idea of the immen- 

 sity of time available for evolutionary changes. 



Even a casual survey of this history — natural history — of the 

 Earth and its inhabitants cannot but impress one with the fact 

 that, taken all in all, there has been a continuous, though not 

 always a uniform, advance in the complexity of organisms from 

 the most ancient times, and that the older types seem gradually 

 to melt into modern forms as the remoter geological eras merge 

 into the more recent. "Only the shortness of human life allows us 



