sistently uniform. That the cytological or cellular evolution is commonly 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.
