CONTENTS IX 



CHAPTER XVIII 



PAGB 



ON SPECIES AND GENUS IN ZOOLOGY AND PALEON- 

 TOLOGY ........ 184-92 



BOOK V 



THE CAUSES OF THE EXTINCTION OF SPECIES 



CHAPTER XIX 



THE LAW OF INCREASE IN SIZE IN PHYLETIC BRANCHES 193-205 

 Generality of the law Special difficulties among the 

 Invertebrates Examples of the law of growth 

 among the lower Vertebrates The law considered 

 as a criterion of the evolution of branches among 

 the Mammals Examples from the Proboscidians 

 and Lophiodonts Is there sometimes a diminution 

 in size ? The dwarf Elephants of the Mediterranean 

 Islands. 



CHAPTER XX 



THE LAW OF SPECIALIZATION OF PHYLETIC BRANCHES 206-16 

 Progressive specialization of Branches Specialization 

 of organs with different functions The runner's 

 foot of the Ungulates The swimming limbs of the 

 Sirenians The natatory paddle of the Ichthyosaurs 

 Production of offensive or defensive weapons 

 Non-functional specializations: line of suture and 

 uncoiling of the Ammonites The Rudistae The 

 senility of Branches. 



CHAPTER XXI 



THE PHENOMENA OF REGRESSION AND OF CONVERGENCE 217-30 

 Regression by parasitism and fixation Rudimentary 

 organs Progressive and regressive evolution 

 Functional and non-functional regression Regres- 

 sive characteristics of the Ammonoids Phenomena 

 of total or partial convergence Conclusions. 



