272 HIGH SCHOOL ZOOLOGY. 



accompanying table shows graphically that the Mesozoic Stiata 

 are about one-ninth the thickness of the Pahieozoic, and it ought 

 to represent the Kainozoic as about one-fifth of the Mesozoic, 

 and the Post-Kainozoic as one-fifth of the Kainozoic, but 

 the exigencies of printing have made the Kainozoic, and 

 especially the Post-Kainozoic, too thick. 



Various calculations have been made as to the absolute time 

 the deposition of the sedimentary rocks has required ; these do 

 not rest on very certain data, but their results come between 

 thirty to sixty millions of years, and are chiefly interesting, as 

 convincing us of the length of time that has elapsed since the 

 appearance of life upon the earth. 



(4) Taxonomic or Classificatory. 



11. Returning to the question of the descent of present forms 

 of life from past forms, it will be observed that it throws a new 

 light on the terms " allied" and " related," so frequently used 

 in the preceding chapter : — the greater or less resemblance of 

 structure, on which classification is based, is due to community 

 of descent, — to more or less distant blood-relationship. 



To facilitate the study of relationship between individuals, 

 genealogical trees ai-e fi-amed on documentaiy evidence, and 

 similar trees have been constructed (going beyond historical 

 times) to show the relationship of nations to each other. The 

 kind of evidence used in the latter case has been jjartly that 

 obtained from observation of the nations at present, (especially 

 the structure of their language, their folk-lore, etc.,) and partly 

 of a palpeontological nature, such as the contents of tombs, and 

 implements, weapons, etc., preserved in the most superficial 

 deposits. 



Going further back, however, into geological time, attempts 

 have been made to construct similar trees, to show the relation- 

 ship of different groups of organic life to each other (phylogeny), 



