THE EXTINCTION OF SPECIES AND OF GROUPS 235 



Trilobites at the end of the Primary, and the Am- 

 monites at the end of the Cretacean. Of course, 

 he essayed to answer this by showing that these ex- 

 tinctions were not so sudden as people tried to make 

 out, and that the vanishing of groups was stretched 

 out over several geological periods. But still it had 

 to be explained why none of these genera or species, 

 with their vast extension, had been able to pro- 

 duce anywhere a descendant capable of surviving, 

 when it is a principle in the Darwinian theory that 

 every organism can and must transform itself, 

 provided the necessary time be accorded to it. 

 The struggle for life is decidedly insufficient to ex- 

 plain the reason of the extinction of species. 



Eminent minds, like those of Quenstedt and 

 Neumayr, struck by these difficulties, had recourse 

 to the very improbable hypothesis of epidemics 

 to explain the phenomena of degeneration, such as 

 the uncoiling of the shells of the Ammonites, 

 closely preceding the extinction of branches. Other 

 naturalists with a more mystical mind have in- 

 voked predestination in the duration of the existence 

 of species, genera, or families. It is curious that 

 this supernatural hypothesis should have, in our own 

 times, found a champion of the weight of Kobelt. 



If it is difficult at the present day to go back to 

 the true causes of the extinction of branches, 

 we are at least beginning to grasp the mechanism 

 or, if it be preferred, the conditions in which the 

 phenomenon is usually produced. Two of these 

 essential conditions are most often united in the two 

 laws of increase in size of the body and specialization 

 of the organism. We are, in fact, permitted by 



