304 cook 



appears to be necessary as well as universal. Free interbreed- 

 ing between the members of large organic groups, or species, is 

 the condition under which biological evolution is going forward 

 in nature, and we have no reason to seek its cause in any aber- 

 ration or specialization of structure or function. 



The fundamental and truly dynamic causes of evolution still 

 lie hidden in the equally unknown causes of genetic variation, 

 but the evolutionary history of a group of organisms is a proc- 

 ess which a kinetic theory adequately explains by supplying 

 physiological reasons and methods. 



The ultimate theory or stage of evolutionary explanation must 

 await far more complete knowledge of the nature of the phenom- 

 ena to which we commonly refer under such abstract terms as 

 matter and force, expressions which we can neither describe nor 

 define, except in a purely formal manner. Much is gained, 

 however, by the recognition of the fact of normal evolutionary 

 motion, by perceiving that organic development is a kinetic 

 phenomenon, for the species no less than for the individual. 

 Individuals and species are conditioned, but not caused, by their 

 environments ; they descend from other species and from other 

 individuals in continuous series of ever-changing forms. There 

 is an inside as well as an outside physiology of evolution, and 

 it is idle to ignore either the one or the other. 



To advance from the static to the kinetic point of view gives 

 us ready and practical solutions for many problems which on the 

 static basis bid fair to have required long periods of time and 

 large expenditure of money. It brings also, as does every 

 advance of science, a host of new questions which the static 

 evolutionist could never have asked, such as the rapidity of 

 evolutionary motion and the means of accelerating, retarding or 

 deflecting it. 



A kinetic theory of evolution does not need to explain varia- 

 tion any more than it needs to explain symbasis and environ- 

 ment ; it accepts these three groups of biological facts, and 

 correlates them as evolutionary factors. Conversely, a theory 

 of variation is not necessarily a theory of evolution ; the two 

 questions may be viewed as quite distinct. The recognition of 

 evolution as a kinetic process does not conflict with a dynamic 



