340 i GÖTE TURESSON 
erect growth and the more contracted inflorescence, also characterize 
the woodland type of the interior. This latter type grows, as has 
been mentioned before, not far from Stenshuvud. It therefore appears 
probable that the Stenshuvud cliff type has become differentiated 
from the local species-population without the intervention of the cliff 
ivpe of the west coast. Different crosses now being made are expec- 
ted to give some information in the near future as to this point. 
4 THE PARALLELISM BETWEEN THE MODIFICATION AND 
THE HEREDITARY VARIATION. 
The morphological parallelism between the modification and the 
hereditary variation offers an additional proof of the control of the 
environmental factors upon the direction of the differentiation pro- 
cess of the habitat types. The structural characteristics brought about 
when a non-halophyte is allowed to develop in solutions of sodium 
chloride, when a Lowland mesophyte is exposed to an Alpine climate, 
or when a land-plant is submersed, are all in the nature of reaction. 
structures arising through reaction to environmental changes. When 
it is found that these same habitats, which lead to modificatory suc- 
culence, dwarfness, etc. in some forms of the species, are habitually 
populated by other forms of the species in which these morphological 
characteristics are hereditary, no doubt would seem to remain as to 
the influence of the habitat factors upon the genotypical composition 
of the species-population present in a certain habitat. That this 
controlling influence may be more or less close has been alluded 
to in the case of the Succisa populations treated above, as well as 
in the treatment of Atriplex and Matricaria. The groups dealt with 
in the first part of this paper furnish, for the rest, ample evidence 
of the parallelism between modifications and hereditary habitat types- 
The fact that in the majority of the cases investigated not the 
habitat modification of the plant but the corresponding hereditary 
type of the species has been found to populate the habitat leads to 
a brief consideration of the theory of adaptive response. If organisms 
are able to respond directly and advantageously to changed conditions, 
we should certainly expect to find that the habitat which calls for 
changed structures on the part of the organisms in order to become 
inhabitable would be populated with organisms thus modified. When 
this only to a limited degree seems to be the case, the course followed 
being rather a genotypical differentiation of the species-population into 
