External and Internal Factors in Evolution 331 



different varieties live together in the same place and there- 

 fore under the same external conditions. It is evident, 

 then, that food conditions have neither originated the dif- 

 ferences nor kept them up. The rarer cases in which in 

 different localities different varieties exist show nothing, be- 

 cause competition and suppression keep certain varieties from 

 developing where it would be possible otherwise for them 

 to exist. 



Nageli says his conclusion may be tested from another 

 point of view. If food conditions, as is generally supposed, 

 have a definite, i.e. a permanent, effect on the organism, then 

 all organisms living under the same conditions should show 

 the same characters. Indeed, it has been claimed in some 

 instances that this is actually the case. Thus it is stated 

 that dry localities cause plants to become hairy, and that 

 absence of hairiness is met with in shady localities. This 

 may apply to certain species, but in other cases exactly the 

 reverse is true, and even the same species behaves differently 

 in different regions, as in Hieracium. And so it is with all 

 characteristics which are ascribed to external influences. As 

 soon as it is supposed a discovery has been made in this di- 

 rection, we may rest assured that in other cases the reverse 

 will be found to hold. We have had, in respect to the influence 

 of the outer world on organisms, the same experience as with 

 the rules for the weather, — when we come to examine the 

 facts critically there are found to be as many exceptions as 

 confirmations of the rule. 



If climatic influence has a definite effect, the entire flora of 

 a special locality ought to have the same peculiarities, but this 

 stands in contradiction to all the results of experience. The 

 character of the vegetation is not determined by the envi- 

 ronment of the plants but by their prehistoric origin, and as 

 the result of competition. Nageli concludes his discussion 

 with the statement that all of our experience goes to show 

 that the effects of external influences (climate and food) 



