CHAPTER XXII 



THE EXTINCTION OF SPECIES AND GROUPS 



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 varia- 

 bility Phases of youth, maturity, and senility of branches 

 Primitive and senile stages in Mammals Recapitulation. 



WE have seen that the evolution of the branches 

 among Fossil animals is regulated by two general 

 laws : that of the increase in size of the body and 

 that of progressive specialization. These data 

 will enable us to approach with profit the interest- 

 ing problem, which has been often discussed, of the 

 causes of the extinction of species and of groups 

 in the course of the geological ages. Nothing is 

 indeed more striking, in following the palseonto- 

 logical history of the globe, than to see the species, 

 the genera, the families, and even the groups of 

 higher order, appear, pass through an evolution 

 with varying wealth of forms, and then decrease and 

 vanish, nearly always with some abruptness. It 

 will be sufficient to recall certain great classical 

 facts. In Primary times, the Graptolites, the Cys- 

 toidea, the Blastoids, the Tetracorals, the Palech- 

 inida, the Clymenise, the Trilobites, the Eurypheridae, 



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