Single varieties of the large and variable genus of _Hieracium_ have lived since the glacial period in the high regions of the Alps, Carpathians, and in the far north, and also in the plains of different geological formations, but these varieties have remained exactly the same, although on all sides there are transitional forms leading from these to other varieties. Some parasitic species also furnish excellent illustrations of the same principle. Besides the several species of Orobanchia and of the parasitic moulds, the mistletoe deserves special mention. It lives on both birch and apple trees and on both presents exactly the same appearance; and even if it is true that mistletoe growing on conifers presents certain small deviations in its character, it is still doubtful whether, if transferred to the birch or apple tree, it would not lose these differences, thus indicating that they are not permanent. It is a fact of general observation that, on the one hand, the same variety occurs in different localities and under different surroundings, and, on the other hand, that slightly different varieties live together in the same place and therefore under the same external conditions. It is evident, then, that food conditions have neither originated the differences nor kept them up. The rarer cases in which in different localities different varieties exist show nothing, because competition and suppression keep certain varieties from developing where it would be possible otherwise for them to exist. Nägeli 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 direction, 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.