28o THE ANIMALS AND MAN 



form only a part of the earth's crust. Rocks that are cooled 

 and solidified from a molten condition, such as volcanic 

 and other igneous kinds, and even stratified rocks that have 

 been highly heated, contain no fossils. Hence the record 

 or history of the life of the long geologic ages before our 

 present one is most incomplete. But it is one of extraor- 

 dinary interest and value in any study of biology. 



Only a very few words can be said about the interesting 

 kinds of earlier creatures, now extinct, that inhabited our 

 earth in ancient times. In the very oldest fossil bearing 

 rocks are found only remains of the simpler kinds of animals 

 as the one-celled kinds, and the sponges, corals, jelly-fishes, 

 etc. In the next oldest strata there are still only simple 

 invertebrate animals but more kinds than in the older rocks. 

 The first vertebrates appear next and these are all fishes. 

 Amphibians are found only in more recent strata, reptiles 

 in still more recent and mammals and birds in still yoimger 

 strata. That is, it is plain from the record of the rocks that 

 the animal types have appeared in succession beginning with 

 the simpler kinds and advancing towards the most complex 

 or higher types by regular stages. This fact is one of the 

 most important that has been learned about life, for it is 

 very strong evidence for the belief of most naturalists that 

 animal kinds are descended from each other, the complex 

 or higher ones from simpler or lower ones. 



The table or diagram on the next page shows the order 

 of the appearance of various kinds of animals in geologic 

 time. 



One must not believe that with the advent of new types 

 of animal life all of the old types became extinct. It is 

 not at all true. Although hundreds of thousands of animal 

 species have become extinct and are known to us only 

 through their fossil remains, or are not known at all, some 



