THE ORIGIN OF SPECIES AND GENERA 277 



tinuous evolution. It must, moreover, be remarked 

 that the explosions of de Vries or Nilsson, interesting 

 as they may be from a biological point of view, go 

 no further than to determine the creation of kindred 

 species so near to each other that no naturalist 

 would dream of grouping them in different genera. 

 Supposing, therefore, that palaeontological evolution 

 has proceeded by skips as modest as those of the 

 (Enotheras or the cereals, we should none the less 

 be compelled, in order to prove the Reptilian origin 

 of the Mammals, to exhume from the Permian 

 or Triassic strata a long series of intermediate 

 genera and species which at the present moment 

 are totally wanting. 



Confining ourselves strictly to ascertained facts, 

 it cannot be said that palaeontology at the present 

 day allows us to specify one single well-demon- 

 strated fact of saltation, or one single series of 

 abrupt changes warranting us in thus explaining 

 the divergence of two genera, of two families, and 

 still less of two orders of fossil animals. There 

 exists, however, a certain group of facts not in- 

 frequently observed, which bring to the hypothesis 

 of abrupt variation at least a certain degree of 

 probability in a few cases. I refer to the inter- 

 mittent tendency shown by branches of producing, at 

 certain moments of their regular evolution, numerous 

 variations round about the parent type, which 

 variations some palaeontologists term varieties, 

 while the majority describe them as distinct 

 species. These periods of crisis, or, if you will, 

 of aberration, in the morphology of certain types 

 generally alternate with relatively calm periods 



