THE ORIGIN OF SPECIES AND GENERA 267 



adhere from choice to the Lamarckian theories 

 founded at once on the action of external conditions 

 and on the mechanical reaction of the organism 

 (use or non-use of the organs, different strains and 

 pressures, etc.) with regard to the environment which 

 surrounds them. But many palaeontologists, rightly 

 struck by the inexplicable facts of the abrupt 

 extinction of whole groups, like the Trilobites, the 

 Ammonites, the Dinosaurs, etc., and by the constant 

 progress of phyletic branches towards an intensive 

 and often exaggerated specialization, wish to add 

 to these causes rather external than anything else 

 of variation, another unknown force of a more 

 internal order, which limits the variation of the 

 groups, as if every one of them at its inception 

 possessed a given amount of sap, the exhaustion 

 of which sooner or later takes place and brings about 

 the fatal extinction of the branch. 



Leaving on one side these burning but difficultly 

 solved problems, we will take our stand on the 

 narrower sphere of palaaontological facts, and at- 

 tempt to fix the visible mechanism of the apparition 

 of fossil forms, or, if you prefer it, the processes 

 worked by Nature in the formation of species and 

 genera, and in the development of new branches. 

 Two opinions have been long since put forth on the 

 mode of the birth of new species. Some see in it 

 the result of slow and gradual modifications accu- 

 mulated by lapse of time. Others, on the contrary, 

 believe in the abrupt and spontaneous apparition 

 of variations, distinct enough at the outset to 

 constitute real species ; this is the hypothesis 

 of abrupt variation or saltation. 



