ESSENTIALS OF A. SYSTEM 17 
to all others is the one that exists between the habitat and the plant. This 
relation has long been known, but its full value has yet to be appreciated. 
It is precisely the relation that exists between cause and effect, and its fun- 
damental importance lies in the fact that all questions concerning the plant 
lead back to it ultimately. Other relations are important, but no other is 
paramount, or able to serve as the basis of ecology. Ecology sums up this 
relation of cause and effect in a single word, and it may be that this ad- 
vantage will finally cause its general acceptance as the proper name for 
this great field. 
In the further analysis of the connection between the habitat and the plant, 
it is evident that the causes or factors of the habitat act directly upon the 
plant as an individual, and at the same time upon plants as groups of in- 
dividuals. The latter in no wise decreases the importance of the plant as 
the primary effect of the habitat, but it gives form to research by making it 
possible to consider two great natural groups of phenomena, each character- 
ized by very different categories of effects. Ecology thus falls naturally 
into three great fundamental fields of inquiry: habitat, plant, and formation 
(or vegetation). To be sure, the last can be approached only through the 
plant, but as the latter is not an individual, but the unit of a complex from 
the formational standpoint, the formation itself may be regarded as a sort 
of multiple organism, which is in many ways at least a direct effect of the 
habitat. In emphasizing this fundamental relation of habitat and vegeta- 
tion, it is imperative not to ignore the fact that neither plant nor formation 
is altogether the effect of its present habitat. A third element must always 
be considered, namely, the historical fact, by which is meant the ancestral 
structure. Upon analysis, however, this is in its turn found to be the product 
of antecedent habitats, and in consequence the essential connection between 
the habitat and the plant is seen to be absolute. 
26. The place of function. In the foregoing it is understood that the 
immediate effect of the‘physical factors of the habitat is to be found in the 
functions of the plant, and that these determine the plant structure. Func- 
tion has so long been the especial theme of plant physiology that methods 
of investigation are numerous and well known, and it is unnecessary here 
to consider it further than to indicate its general bearing. The essential 
sequence in ecological. research, then,.is the one already indicated, viz., 
habitat, plant, and formation, and this will constitute the order of treatment 
in the following pages. That portion of floristic which is not-mere de- 
scriptive botany belongs to the consideration of the formation, and in con- 
sequence there will be no special treatment of floristic as a subdivision of 
ecology. 
