THE EVOLUTIONARY SIGNIFICANCE OF SPECIES. 405 



interbreeding of nonnally diverse individaids (symbasis) weaves 

 new types out of the variations of many lines of descent. 



All organisms are.siil)ject to selective influence in the sense that 

 variations are rejected with a promptness proportional to their harm- 

 fulness in the given environment, l)ut generally this leaves a very 

 wide latitude of ])ossible changes in which selection does not interfere. 

 The instances are relatively rare in. which existence becomes acutely 

 dependent upon the development of some one characteristic or quality, 

 and such narroAv selection does not strengthen the type, but insures 

 and hastens its extinction.'* 



The neglect of this distinction vitiates much evolutionary literature, 

 both that which treats selection as an actuating " force " and that 

 which rejects selection for " discontinuous variation " or '' the muta- 

 tion theory."" It is true that many variations of inbred domesticated 

 plants and animals are very abruptly discontinuous, and that such 

 changes are not caused by selection, but these facts in no way mili- 

 tate against others equally obvious, that the natural evolution of 

 new types is a relatively' slow and gradual process, and that selection 

 influences the direction of this continuous vital motion. The older 

 selective hypothesis was only half erroneous. Selection does not set 

 stationary organisms in motion, but it often guides sj)ontaneous 

 change. It does not explain evolution or vital motion in general, but 

 it does explain adaptation, or motion in some particular direction, as 

 when one species differs from its relatives in special characters which 

 enable it to exist in a special environment. That all adaptations are 

 mere coincidences is as improbable as that all characters represent 

 useful adaptations. 



Selection is not, as many " Darwinians '' have maintained, the true, 

 efficient, cause of evolution. The vital motion of species proceeds 

 whether selection is operative or not. Species do not acquire char- 

 acters from the environment, l)ut merely in accordance with it. At 

 any point in the evolutionary journey selection may determine 

 wdiether certain characters shall be acquired or not. It is an obstacle 

 in the environmental road over which the species would travel, instead 

 of being the source of power of the organic automobile. Selection 

 prevents motion in one direction, but permits advance in another. It 

 explains why a particular character becomes accentuated in a par- 

 ticular species, but is no more a cause of the developmental progress 

 of the species than the turns of the road are the motive power of the 

 vehicle. 



The hypothesis of selection as the active principle or causal agency 

 of evolution became illogical and useless as soon as the inheritance of 

 acquired characters was discredited. The first idea without the 



oO. F. Cook, 1903, " Stages of Vital Motion," Popular Science Monthly, 63: 16. 



