982 The American Naturalist. [November, 
to all minor influences. That portion of the paper dealing with the 
physical conditions of the locality is the most important, since it sug- 
gests methods of procedure for other regions. It deserves study at the 
hands of all who are engaged with similar problems. But, at the same 
time, it must be said that for most purposes the analysis will be found 
too exhaustive. The perception of the relation of habitat and organ- 
ism stands at the foundation of phytogeography ; yet all of the physical 
factors are not of equal importance. Plants have more to do than 
merely to contend against physical conditions; they have to contend 
with other plants. Individuals contend with each other, vegetation 
form with vegetation form, floral element with floral element, and 
formation with formation ; a struggle goes on everywhere in the floral 
covering that forces plants to adapt themselves to strange and even 
unwelcome environments. The influence of modifications due to this 
necessity is one that may never be overlooked. Extended investigation 
of a large territory is required to enable one to measure the relative 
importance of these factors, but it cannot escape notice that a number 
of species listed by Professor MacMillan are ruderal species of varied 
habitat, which may be met with in meadow, clearing, high prairie or 
sand hill, as well as in the situations where he found them. In fact, it 
is only by beginning at the other end, by studying the vegetation forms 
of a region and the accessory characters of this vegetation, and so 
reaching an understanding of the means by which plants are enabled 
to take possession of the soil and to maintain themselves there, that 
one can comprehend the really characteristic and dominant vegetation 
and arrive at an understanding of formations. 
The discussion of the biological factors which determine the vegeta- 
tive covering of the region, therefore, seems quite inadequate, An 
analysis of these biological factors made with the same minuteness as 
that of the physical factors, might not be necesssary, and yet it cannot 
fail to suggest itself to those who have been engaged in the study of 
like problems in other regions, that the former, in general, is likely to 
be the much more valuable, if carried beyond a certain point. The phy- 
siographical and climatological conditions are undoubtedly of control- 
ling force while vegetation is establishing itself in a new area. Once 
established, vegetation reacts upon its environment and upon itself, 
and new forces have to be reckoned with. One would think that a 
more careful consideration of these and of the influence of vegetation 
forms and their distribution would bave made such phenomena, as the 
occurrence of Celastrus and Parthenocissus in the back-strand rather 
than in the mid-strand (p. 979} much more clear. 
