External and Internal Factors in Evolution 327 



Organisms, as has been said, are distinguished from one 

 another, not only in that one is simpler and another more 

 complicated, but also in that those standing at the same stage 

 of organization are unequally differentiated in their functions 

 and in their structure, which is connected primarily with cer- 

 tain external relations which Nageli calls adaptations. 



Adaptation appears at each stage of the organization, which 

 stage is, for a given environment, the most advantageous 

 expression of the main type that was itself produced by 

 internal causes. For this condition of adaptation, a suffi- 

 cient cause is demanded, and this is, as Nageli tries to show 

 later, the result of the inherited response to the environ- 

 ment. In many cases this cause will continue to act until 

 complete adaptation is gained ; in other cases, the external 

 conditions give a direction only, and the organism itself con- 

 tinues the movement to its more perfect condition. 



The difference between the conception of the organic king- 

 dom as the outcome of mechanical causes on the one hand, or of 

 competition and extermination on the other hand, can be best 

 brought out, Nageli thinks, by the following comparison of 

 the two respective methods of action. There might have been 

 no competition, and no consequent extermination in the plant 

 kingdom, if from the beginning the surface of the earth had 

 continually grown larger in proportion as living things 

 increased in numbers, and if animals had not appeared to 

 destroy the plants. Under these conditions each germ coiud 

 then have found room and food, and have unfolded itself 

 without hinderance. If now, as is assumed to be the case 

 on the Darwinian theory, individual variations had been in 

 all directions, the developmental movement could not have 

 gone beyond its own beginnings, and the first-formed plants 

 would have remained swinging now on one side and now on 

 another of the point first reached. The whole plant kingdom 

 would have remained in its entirety at its first stage of evolu- 

 tion, that is, it would never have advanced beyond the stage 



