BOOK V 



THE CAUSES OF THE EXTINCTION OF SPECIES 

 CHAPTER XIX 



THE LAW OF INCREASE IN SIZE IN PHYLETIO 

 BRANCHES 



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. 



SEVERAL times in the course of the chapter de- 

 voted to the study of the phyletic branches, I have 

 had occasion to notice, in passing, the gradual 

 increase in size of the mutations of the same branch, 

 rising from the earliest to the latest forms. This 

 law, which we will call simply The law of in- 

 crease of size in phyletic branches, is one of the 

 most curious and, from its generality, most im- 

 portant, which has been brought to light by the 

 researches of modern palaeontologists. It is ob- 

 served almost without distinction in all classes of 

 the animal kingdom, but presents more numerous 

 and clearer applications in the group of the Verte- 

 brates than in that of the Invertebrates. Conditions 

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