382 PRACTICAL LESSONS IN SCIENCE. 



adapted themselves to changing conditions and crowded into 

 every new addition to the domain of life. As conditions changed 

 some forms could not adapt themselves, and, overcome in the 

 struggle, passed away and others took their places. It is esti- 

 mated that there were more than thirty complete changes of 

 species during the paleozoic time. So that the principle of the 

 " survival of the fittest " seems to have prevailed from the very 

 beginning of life on the earth. 



The graphite of the early ages, the limestones of the Silurian 

 and later ages, the coal of the carboniferous, all represent impuri- 

 ties removed from air and water by different forms of life. It is 

 interesting to note that each form of life has helped to prepare 

 the way for other and higher forms, generally lessening the op- 

 portunities for their own existence. The coal plants not only ren- 

 dered the air more pure, but so changed it as to allow the sunshine 

 to reach the surface of the earth with vigor enough to promote 

 the growth of flowering plants. 



Every step in the history of the earth prior to the advent of life 

 was in preparation for life, and every step afterward was in prep- 

 aration for higher forms of life. 



But the geological record at present is by no means contin- 

 uous; the Wank between the archaean and the Cambrian equals 

 nearly one third of the whole record, leaving the origin of the 

 protozoa, coelenterata, arthropoda, echinodermata, mollusca, 

 molluscoida and tunicata without record. Some of the rocks of 

 this period have been found, and some fossils have been discov- 

 ered, but the rocks are so highly metamorphic that there is little 

 hope of ever finding anything like a complete record of the period. 

 In this period the protophyta, zygophyta, oophyta, and carpo- 

 phyta had their origin, of which no trace has been found in the 

 rocks. 



Fishes, the first of the vertebrates, appeared somewhat ab- 

 ruptly in the later part of the Silurian age. It is claimed that 

 the first so-called fiiJies were of a comprehensive type, from which 

 both fishes and reptiles were derived, but transitional forms are 

 not numerous. And so on, the geological record inageneral way 



