THE EVIDENCE FOR ORGANIC EVOLUTION 



509 



the " leaf impressions " (Fig. 266 A) that are often seen in collec- 

 tions of fossils. Insects have sometimes been preserved in almost 

 perfect condition after being caught in resins of trees, when this 

 material became hardened into a more permanent substance, as 

 with the insect-containing amber of the Baltic region. Animals 

 of the Glacial Period were sometimes frozen in the ice and snow and 

 preserved for many thousand years, as was the mammoth found in 

 Siberia (Fig. 272). The peat bogs of the north and the tar pools of 

 Southern California contain 

 the skeletons of animals that 

 ventured upon the edges of 

 the treacherous surface and 

 sank to their death. But 

 these are exceptional cases as 

 compared with the great 

 majority of fossils, which are 

 portions of the hard parts of 

 animals embedded in solid 

 rock (Fig. 266 C). 



Although there is a wealth 

 of fossils, the incompleteness 

 of the fossil record must be 

 recognized. Not only are the 

 great majority of fosrsils merely 

 fragments, but the known 

 species cannot include more 

 than the merest fraction of the animal and plant species that 

 have lived, because the chances are very small that the re- 

 mains of any organism will ever l^ecome fossihzed. Sometimes, 

 however, skeletons are found that are almost complete (Figs. 267 

 and 268) and the record as a whole seems to give a fairly repre- 

 sentative picture of the life existing in the past. In a famous 

 chapter of " The Origin of Species," entitled " The Imperfec- 

 tion of the Geological Record," Darwin compared this record 

 with a book which had been mutilated by the removal of many 

 whole pages and the injury of others, but in which one could still 

 trace the later course of the story. 



The Geologic Successioji . — It is necessary to explain certain 

 principles, used by geologists in their study of the rock layers, if 

 one is to understand the evidence for a succession of animal types 



Fig. 271. — Eggs ul' ;i dmusaiir, wall 



matrix of stone partially removed, as 



found in the Gobi Desert, Mongolia. 



These extinct reptiles (r/. Fig. 269) pre- 

 sumably laid eggs like existing reptiles. The 

 discovery of such eggs by the Third Asiatic 

 Expedition of the American Museum of 

 Natural History proves this in the case of one 

 species. In one instance the remains of a 

 dinosaur embryo were discovered in an egg. 

 (From Report Field Museum of Natural 

 History, Vol. VII.) 



