PHYLOGENETIC RELATIONS OF ANIMAL GROUPS 873 



Geological Evidence.— Paleontology is the study of fossil remains 

 of organisms deposited in the strata of the earth's crust. Shells 

 and other hard parts are mineralized or petrified; in other cases 

 mud impressions, or tracks, or pitch-preserved individuals, such as 

 insects, and a few frozen forms constitute the majority of fossils. 

 It must be kept clearly in mind that the geologist is able to deter- 

 mine quite accurately the sequence of time or chronological suc- 

 cession of the layers of the earth's crust. The geological time scale 

 shows a long period before any life existed, then the appearance of 

 unicellular plants, then unicellular animals, then colonial forms, 

 simple many-celled forms, and then the more complex ones. Such 

 a timetable estimates the relative period of time during each era 

 and shows some fusion and overlapping of certain types of life. 

 Certain types of unicellular forms are continuous through the en- 

 tire scale. 



The principal facts shown by the fossil record may be summa- 

 rized : (a) The fossil forms are not strictly identical with any living 

 species, and the remains of plants and animals of each geologic 

 stratum are at least specifically different from the forms in any 

 other stratum, but they may belong to the same genus; (b) the 

 oldest strata containing fossils have represented in them most of 

 the simpler forms of nonchordate animals, while the upper strata 

 contain fossils of all groups more nearly like modern forms, includ- 

 ing chordates; (c) in studying these in sequence, there may be 

 observed a gradual progression from simpler and generalized types 

 toward more specialized and complex forms as one proceeds from 

 the older toward the upper or newer strata; (d) only the more gen- 

 eralized varieties have persisted within the groups that, as a whole, 

 have become specialized ; many of the others have long since reached 

 their climax of specialization and have become extinct; (e) many 

 of the dominant groups of organisms have arisen near the close of 

 a period during which great climatic changes were taking place and 

 have enjoyed dominance during the following period because such 

 a group probably arose in response to the conditions; (f) although 

 many nonchordate phyla had reached an advanced stage of devel- 

 opment in the early Cambrian period, where early fossil records occur, 

 many ancestral sequences have been observed, and these have sup- 

 plied information making possible the detailed description of the 

 course of events that has led to the surviving animals of modern 



