Divisions of Life and Time 



their remains would be preserved and subse- 

 quently found. Birds, on the other hand, are 

 extremely rare, particularly the earlier species, 

 which are the ones we are most anxious to 

 know. It is commonly stated that this is on 

 account of their power of flight, as well as the 

 lightness of their bodies, the first preserving 

 them from many accidents to which other ani- 

 mals are subject, while the last caused their 

 bodies to float and rendered them particularly 

 liable to destruction after death. Still this ex- 

 planation is not quite satisfactory, for birds 

 sometimes perish in great numbers in spite of 

 their power of flight, and in some favored 

 localities many of their bones are found. 



As charity covers a multitude of sins, so the 

 term invertebrates includes a vast number of 

 animals which agree with one another in the 

 negative character of lacking a back-bone. 

 Formerly the invertebrates were regarded as 

 forming a group of equal rank with the verte- 

 brates, the two divisions including all animal 

 life ; but it is now known that this assemblage 

 comprised several distinct classes of animals, 

 4 41 



