X THE TRANSFORMATIONS OF THE ANIMAL WORLD 

 CHAPTER XXII 



PAGE 



THE EXTINCTION OF SPECIES AND GROUPS . . . 231-43 

 Sudden disappearance of groups Groups extinct and 

 evolved Causes of extinction Weakness of the 

 Darwinian hypothesis Extinction by gigantism 

 Laws of Dollo on the irreversibility and the limitation 

 of evolution Progressive reduction of variability 

 Phases of youth, maturity and senility of branches 

 Primitive and senile stages in Mammals Recapitu- 

 lation. 



BOOK VI 



THE MECHANISM OF THE PRODUCTION OF NEW FORMS 



CHAPTER XXIII 



THE LAWS OF CONTINUOUS PROGRESS AND THE APPEAR- 

 ANCE OF GROUPS 244-52 



Law of the late appearance of the higher types Dis- 

 coveries making against this law The epochs of the 

 first appearances of groups found to be more and 

 more remote. 



^ ] 



CHAPTER XXIV 



INDIVIDUAL AND PAL^EONTOLOGICAL EVOLUTION : ONTO-, 



GENY AND PHYLOGENY ..... 253-64 



The great biogenetic law of Haeckel Embryological 

 acceleration or tachygenesis Embryonic types per- 

 sisting in the fossil state Study of individual de- 

 velopment in the Ammonoids and Lamellibranchs 

 The milk teeth of Mammals. 



CHAPTER XXV 



THE ORIGIN OF SPECIES AND GENERA . , . . 265-79 

 Our ignorance of first causes Two hypotheses : slow 

 and abrupt variation Slowness of the direct or 

 normal evolution Divergence through geographical 

 isolation Abrupt variation or saltation of de Vries 

 Its application to fossil animals Conclusions. 



