56 



GENETICS 



when they throw off their progeny, as on a tangent. 

 They finally pass into the excessively differentiated 

 period of old age, from which there is no recall, al- 

 though they approach in many features the infantile 

 condition, and end in death or extinction. This cycle is 

 repeatedly illustrated by phylogenetic lines of fossil 



forms which have 

 long since become 

 extinct. 



Beecher has 

 pointed out that, 

 in paleontolog- 

 ical times just 

 before they be- 

 c a m e extinct, 

 species often un- 

 derwent extreme 

 Fio. 11. Diagram of the relation of re- ... 



production to the life-cycle. specialization in 



the form of fan- 

 tastic shapes, an excessive number of spines or elabo- 

 rate sculpturings on the shells as seen among the 

 ammonites, belemnites, and trilobites, or of gigantic 

 size as in the dinosaurs, plesiosaurs, and theromorphs. 

 All of these facts indicate a species-cycle in which these 

 abnormal features were the unmistakable signs of old 

 age. 



The reproductive period of a species when mutants 

 are being thrown off, as of an individual, may extend 

 over a considerable period of the whole cycle, or it 

 may be confined to a relatively small segment. It is 

 possible that in the evening primrose deVries may 



