THEORY OF EVOLUTION 



491 



the simple 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 ob- 

 served 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 



Wr/st 



Wrist- 



Fig-. 210. — Positions of the human hand to show the comparative stages of 

 elevation of the horse's foot to the tip of the middle toe. (Courtesy of American 

 Museum of Natural History.) 



a period during which gieat 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 develop- 

 ment 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 

 times; (g) the developmental changes of the chordate groups are 

 more completely read in the fossil record, with the history of the 



