PROLONGED CONSTANCY IMPEDES RAPID READAPTATION 153 



Nautili made their appearance. It is very probable tbat the 

 extiaction of the Trilobites stands in direct relation to the 

 evolution of the Nautili. It may be said that the Trilobites 

 succumbed because they were attacked by senile decay, that 

 they were too old to resist the assaults of younger and more 

 vigorous enemies. But, on the contrary, the extinction of the 

 Trilobites is certainly due to their failure to adapt themselves 

 to new and modified conditions. Greater swiftness, more rapid 

 multiplication, and greater intelligence on the part of the NautUi, 

 were responsible for the extinction of the Trilobites. The latter, 

 long adapted to constant conditions, were unable to adapt 

 themselves with sufficient rapidity to the suddenly modified 

 environment ; and this failure to adapt themselves was due, 

 not to senile decay, but to the fact that, in old and constant 

 species, the determinants of the species are in a majority in all 

 the ids of the germ-plasm, which is consequently unable to change 

 its constitution except very slowly and gradually. The environ- 

 ment, in a word, changed too rapidly for the determinants of 

 the long-adapted germ-plasm to keep pace with it. 



In a great many cases the extinction of species may un- 

 doubtedly be attributed to the fact that the necessary variations 

 involved in readaptation do not come about quickly enough. 

 In other cases such extinction may be due to simple and direct 

 inferiority in the struggle for existence ; and this is the case 

 with all those species which formerly existed wild in Europe, 

 the wolf, the bear, the lynx, etc., which have now been nearly 

 completely exterminated by man. Or, again, in other cases 

 it may be due to a change in the flora of a region — as, for instance, 

 in St. Helena, where the destruction of the dense forests which 

 used to cover the island brought in its train the extinction of 

 the endemic species of that island. To such an extent has man 

 persisted in the destruction of wild animals that European 

 Governments have been compelled to take steps to secure a 

 " close season " for the girafie, the lion, and other wild animals 



