284 COOK 



merely inadequate as a cause of evolution ; it is not an evolu- 

 tionary cause at all, but only a test and an evidence of the effi- 

 ciency of other causes which reside in the species and enable it 

 to go forward with persistence, even when obliged to follow a 

 narrow path between environmental obstacles. 



Selection is potent to explain the further extension and devel- 

 opment of favorable variations only by its ability to influence an 

 evolution which is already in progress, and not in any sense 

 which renders it a cause of evolution. The selective potency 

 of the environment consists only in its ability to restrict evolu- 

 tion, not in any power to actuate or to carry forward the process 

 of development. Selection may still be enumerated as an evo- 

 lutionary factor, but it is wholly a negative factor, restrictive 

 and not constructive. 



DARWINIAN FORMULAE OF EVOLUTION. 



Evolution is a name for the process of gradual change by 

 which the diversity of organic nature has come about. Darwin's 

 theory of natural selection was based on the indication that some 

 of the characters of plants and animals have been attained 

 because individuals possessing these characters had an advan- 

 tage in the struggle for existence. Many Darwinians " more 

 Darwinian than Darwin " have made this proposition universal 

 and say in effect that all characters of plants and animals have 

 arisen because they give or have given their possessors advan- 

 tages in the struggle for existence. 



Darwin's original proposition points in the direction of an im- 

 portant truth, that plants and animals are specially adapted to 

 their various environments. Great emphasis came to be placed 

 on this point because the adjustment of species to their respec- 

 tive places in nature had been taken to prove the special crea- 

 tion of species, so that a theory of gradual development had to 

 supply a solution for the problem of adaptation before it could 

 expect to receive general credence or even the serious con- 

 sideration of the scientific public. 



In the course of the discussion which raged in the decades 

 after the publication of the Origin of Species attention was prin- 

 cipally directed to the phenomena of adaptation and speciation, 



