THE GENOTYPICAL RESPONSE OF THE PLANT 341 
different, hereditary habitat types, the advantage of such responses on 
the part of the individual becomes highly questionable. The conclu- 
sions to be drawn from the data presented in this work point rather 
to an interpretation of the habitat modification opposite to the theory 
of adaptive response. 
The question may be made clearer by the assumption that the 
same characteristic which in one form of the species (the resulting 
modification) requires the exposure to an environmental factor of 
high intensity in order to become developed, may in another form 
(the hereditary variation) result as a response to a very much lower 
intensity of this factor (Turessox. 1919 a). It is conceivable that the 
habitat factor responsible for the development of the characteristic 
in question may at the same time act as a limiting factor upon the 
general development in the case of the modification, while no such 
limiting action results in the hereditary variation because of the 
promptness with which it responds to this same habitat factor. The 
readiness with which an extreme habitat so often-is populated by the he- 
reditary habitat type to the exclusion of the parallel habitat modification 
indicates precisely this nature of the »adaptiveness» of the both forms. 
The general results obtained by Küösrer (1916), Derro (1904) and 
other investigators, who have attacked the problem from other aspects, 
seem to be consistent with the view developed above. 
II. THE SPECIES AND THE VARIETY AS ECOLOGICAL 
UNITS. 
It should not be thought that the differentiation of a species- 
population into hereditary habitat types is a phenomenon peculiar to 
the species discussed above. The cultivations hitherto made of other 
species, including for instance Rumex acetosa L., Solidago virgaurea 
L., Artemisia campestris L., Campanula rotundifolia L., Ranunculus 
acer L., indicate precisely the same behaviour. The same will very 
likely be found to hold true for the majority of common plant species. 
It is in fact to be assumed that the rarity of certain species is in great 
measure due to a decreased power of genotypical response to habitat 
differences, climatic and edaphic, within their area of distribution. 
Thus, as a result of genotypical responses of the species-popula- 
tion to different habitats, isolated units are formed within the species 
much in the same way as contemplated by Jorpax (1905) and 
Hacrepoorn (1921). However, to speak of such units as »species», 
