THE SPECIES AND THE VARIETY AS ECOLOGICAL UNITS 111 
of variety present in a certain locality is mainly determined by chance. 
The intimate relation of the above discussed varieties to the environ- 
ment is, indeed, the only possible explanation. It is for several rea- 
sons inappropriate, however, to employ the terms adaptation or adap- 
tive forms for these kinds of varieties. It introduces and propagates 
the teleological idea that need controls the origin of characters. The 
term likewise brings with it the erroneous idea that the origin of so- 
called adaptive characters represents a phenomenon sui generis, viz. 
the origin of these characters through the direct (Lamarckian) effect 
of environmental influences, in contrast to the origin of so-called 
»Organisationsmerkmale». The concept of adaptation, further, has led 
biologists to believe that the primary cause enabling a certain plant 
to live in an extreme locality is to be sought for in the habitus of 
that plant and in morphological details. However, it is evidently the 
genotypical constitution of the individual plant which is the point of 
primary importance. The morphological details and the habitus of a 
given plant represent only the result produced by the reaction of the 
genotype to definite external conditions, it represents the reaction- 
type‘. We further know that similarity in reactiontype does not 
mean similarity in genotype. The prostrate character in certain Atri- 
plex forms, for instance, may be in the one case a modification, in 
the other case a hereditary variation, for even some of the usually 
erect branched forms may become prostrate if the intensity of light 
becomes strong enough, as I have shown some years ago (TURESSON, 
1919). That the process of selection, however, intervenes and directs 
the genotypical differentiation of the population in a given locality 
becomes apparent when it is found that the majority of the indivi- 
duals on the exposed west coast consist of hereditary prostrate forms, 
i. e. forms reacting very readily to light and becoming prostrate even 
in ordinary light. 
The differing phenotypical reactions, or the different reaction- 
types, of a given plant, when exposed to differing environmental fac- 
tors, are thus factors of great moment. However, the differing geno- 
typical reactions of a plant population, when distributed over a con- 
tinuous area comprising different types of localities, is a fact of still 
greater importance from an ecological point of view. From what has 
been said in regard to the occurrence of distinct varieties of an eco- 
1 Used here in the same sense as phenotype (JOHANNSEN, 1913); the term 
might have some value when the product of the reaction is to be emphasised. 
