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 cou.d 
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 
