24 POPULAR SCIENCE MONTHLY. 



sistently uniform. That the cytological or cellular evolution is com- 

 monly slower than that which affects external characters seems probable 

 because domesticated plants and animals more different than mutually 

 sterile wild species are still completely fertile. That all the types 

 produced under domestication from the same wild species hybridize 

 freely, and thus do not have the stability and isolation of natural 

 species, was frankly admitted by Darwin and Huxley as 'one of the 

 greatest obstacles to the general acceptance and progress of the great 

 principle of evolution,' and it is no less an obstacle to the acceptance 

 of the complicated and self-contradictory static theories formulated as 

 alleged improvements of the views of these evolutionary pioneers. If, 

 however, evolution be recognized as a kinetic process this fundamental 

 difficulty completely disappears, since the cross-fertilization which 

 hinders the segregation of species is not on this account an obstacle 

 to evolution, but is, on the contrary, the most important agency for 

 the acceleration of vital motion. By overlooking this fact builders of 

 evolutionary theories have continued, as it were, to stumble over the 

 corner-stone of the biological structure. 



