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AN INTRODUCTION TO MODERN GENETICS 



Hyatt placed great emphasis on palingenesis in evolution; that is to 

 say, on the theory that the early development of an organism sum- 

 marized the complete evolutionary history of its ancestors (recapitu- 

 lation), and that the evolutionary advance appeared as something 



Fig. 108. Some Ammonites to Illustrate "Programme Evolution" in the 



Group. — A. An early straight form with simple suture line (Bactrites, Upper 

 Devonian). 6. Early loosely coiled, simple sutured form (Clymenia, Upper 

 Devonian). C. A typical middle form (Stephanoceras, Jurassic). D. Tightly coiled 

 form v/ith complex suture (Phylloceras, Jurassic). £. The suture line at its most 

 complicated; the space between two successive lines has been painted black. 

 The drawing shows only one side of the shell, the mid-line being to the right 

 (Pinacoceras, Trias). F. An uncoiled late form with simplified suture (Hamulina, 

 Chalk). G. A form with helical coiling such as is found in the latest species of 

 the group; the form figured, however (Cochloceras), is from the Upper Trias, 

 and shows that these types occurred in the heyday of the group and not only 

 in its "senescent" phase. 



(After Zittel.) 



