Animals J2efore Man 



of these colonies, for the chances are that it may 

 contain some hitherto unknown species. 



And here we may bid farewell to the in- 

 vertebrates that peopled the waters of the an- 

 cient world, for although one more system of 

 rocks, the Permian, is included in the great 

 Paleozoic group, yet historically it may best 

 be considered in another place, for its life marks 

 an advance over that contained in the rocks 

 below by the introduction of creatures higher 

 in the scale. 



It was said in a previous chapter that no 

 satisfactory answer cou-ld be given to the ques- 

 tion, " How long ago did that animal live ? " but 

 still we may be permitted to make a guess at 

 this as well as at the length of any given por- 

 tion of the world's history. Now, the part we 

 have just been considering, beginning with the 

 life of the Cambrian and ending with the Car- 

 boniferous, has been estimated to represent a 

 duration of something like 12,000,000 years. 

 This estimate may be said to have a very solid 

 foundation, for it is based on the thickness of 

 rocks included in the Paleozoic group, and this 



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