THE GENOTYPICAL RESPONSE OF THE PLANT 333 
theory a basis for the explanation of the facts. From an ecological point 
of view the arguments already raised against such a theory by NAGEL 
(1866) may still be considered valid. We have found that the habitat 
type — even if it may appear to be quite homogeneous in its habitat 
__ is made up of a number of individuals of which — in the case of 
allogamous plants — none may represent the genotype of another. 
These individuals are nevertheless exposed to the same environmental 
factors and should in time become identical with one another, according 
to the Lamarckian view. NAGreLI denies — and surely upon good 
grounds any such effect of the environmental factors. NAGELI says 
further that when for some reason two varities are each confined 
to a special habitat, there is not found an intermediate variety in the 
transitional area between the two habitats, which most likely would 
have been the case had the varieties been direct products of the habi- 
tat. The force of this argument has been especially appreciated by 
Bateson (1913). It applies with equal strength to the Hieracium po- 
pulation in the transition area dealt with above, and will probably 
also be found to hold good in the great majority of cases which involve 
the distribution of »climatic varieties», as is repeatedly maintained by 
BATESON. 
When the Lamarckian view must thus be discarded as an expla- 
nation of the characteristics which, in spite of the hereditary differen- 
ces present, are common to the separate individuals of one and the 
same habitat type, reference must be made to the Darwinian theory 
of selection. The attempt to make selection responsible for the defini- 
teness of a certain type has been made over and over again. We know 
now for certain, however, that definiteness of a type does not necessa- 
rily mean that the special characteristics exhibited by the type in 
question have made possible or have contributed to the survival of that 
type in a certain habitat. When a few individuals or a colony become 
isolated and remain isolated from intercrossing with the multitude 
of the species-population, the chances are that these isolations will in 
time exhibit peculiarities of characters not found in the rest of the po- 
pulation. For a full account of the problem reference should be made 
to Bateson (1913). Illustrative examples of the origin of »species» 
due to the existence of barriers to intercourse are furnished by the 
Hawaian snails of the genus Achatinella (GuLick, 1905), by the aberrant 
rat colonies which Lioyp (1912) found confined to isolated buildings 
in the towns of British India, or again by the aberrant rat populations 
