348 ANIMAL BIOLOGY 



of the tongue is the homologue of part of the bony frame- 

 work of gill arches found in the fishes. 



The student of the development of life naturally looks 

 with interest upon the revelations of geology as to the 

 succession of organisms on the surface of the earth. The 

 earth itself has undergone an evolution from a relatively 

 homogeneous condition in which it was a heated mass of 

 molten rock. With the cooling and thickening of the 

 crust there came the condensation of water which formed 

 shallow seas covering most if not all of the surface. As 

 the earth cooled further its surface was thrown into folds 

 which gave rise to mountains. With the elevation of 

 land there began the process of erosion and the deposition 

 of sediment in the bottom of the primitive seas. Thus 

 were produced the stratified rocks which have been slowly 

 formed to a thickness of many miles. Subsequently 

 many of the strata deposited at the bottom of the sea 

 were raised up, thus affording to the geologist an opportu- 

 nity to study the fossils or remains of living forms which 

 they contain. It has taken a long time to trace out the 

 succession of strata that are found, some here and some 

 there, over the earth's surface, but the labors of many 

 geologists have now given us a fairly adequate account 

 of the history of the earth and its inhabitants. These 

 rocky strata are the leaves of a great book in which the 

 earth has written its own history. Beginning with the 

 lowest and the earliest strata in which remains of living 

 forms occur we can follow the successive stages in the 

 evolution of life as we pass to more recent times. Most 

 of the earliest records of life have been obliterated, but in 

 the Cambrian period living forms are preserved in great 

 abundance. Most of the phyla of invertebrate animals 

 were represented, and there were several groups, such as 

 the trilobites and graptolites, which have long become 



