STUDIES ON THE VEGETATION OF ICELAND 159 
of the Icelandic species, partly for the question of the cultivation 
of the various Icelandic types of vegetation. 
Other questions of decisive importance in studies on the di- 
stribution of the species in the scales of external factors are partly 
the question of equidistance in division, and partly the question of 
the determination of the number of external factors bearing on plant 
distribution. 
As far as the first question is concerned, in formations with 
low density of species, the line between two formations is most 
naturally drawn at the physiognomie boundary line between the 
two formations, and the areas selected for examination should as 
far as possible be laid in the middle of the formation. It is pos- 
sible that the distance between the various localities examined will 
not in this way become an exact expression of physical equidistance 
between the localities, or the formations, but merely of ecological 
equidistance; but since the investigation is primarily ecological, it 
will suffice if the requirement of ecological equidistance is satisfied, 
even though physical equidistance would have been desirable. 
Where we are concerned with the investigation of formations 
with many species, the requirement of ecological equidistance be- 
tween the localities examined will be considerably more difficult 
to satisfy. The present treatise deals principally with formations 
of this kind, and the examination of them was made in the follow- 
ing way. On a gently sloping surface the investigator passed so 
far up and down from one locality that the vegetation had changed 
appreciably; the second locality was then examined here, whereupon 
the third locality was chosen and examined in the same way. 
It is possible that the distances between the localities examined 
are unequal both physically and ecologically; so much is certain, 
however, that the sequence of the localities examined expresses a 
constantly increasing change of environment. If this is the case, 
however, we have in the proportion of the species points 
of species occurring principally above, and species oc- 
curring principally below, the formation in question an 
aid in determining the question of ecological equidi- 
Stance between the formations. 
Another question of equal importance is the question of the 
determination of the number of plant-distributing factors. This 
question, however, is only topical in a plant covering rich in species 
and of uniform physiognomy. If such a plant covering is examined 
