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, z.¢. 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 Mzeracizum. 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 allthe 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) 
