THE EXTINCTION OF SPECIES AND OF GROUPS 243 



of the evolution being essentially variable according 

 to the group. Thus the Hyrax or present Daman 

 possesses an entirely primitive skull, comparable, 

 in degree of evolution, to those of several small 

 Ungulates of the Eocene period. It will be seen 

 what grave errors one would be led to commit 

 did one attempt to use these stages of evolution, 

 as Gaudry proposed, as the sole criterion for deter- 

 mining the absolute age of fossil animals. 



Thus the general evolution of the animal king- 

 dom presents itself to us as being constituted by 

 a sheaf of innumerable phyletic branches parallel 

 in their evolution, and without ever having inter- 

 mingled during a more or less long series of 

 geological periods. Each of these branches arrives, 

 with more or less speed, at mutations of great size 

 and of very specialized characteristics, which 

 vanish without leaving descendants. When a 

 branch disappears by extinction, it is, so to speak, 

 replaced by another branch having an evolution 

 until then slower, which in its turn passes through 

 those phases of maturity and old age which conduct 

 it to its end. The species and genera of the present 

 time represent those which have not yet arrived 

 at the senile phases ; but it can be foretold that 

 some among them, the Elephants, the Whales, the 

 Ostriches, etc., are approaching this final phase of 

 their existence. The mechanism of the extinction 

 of species commences, therefore, to show itself 

 with a certain clearness. We should now ask our- 

 selves how much knowledge we have of the opposite 

 side of the problem of life, I mean of the origin of 

 species and groups of a higher order. 



