195 
The Problems of Ecology. 
Now it is the study of such plant-associations, the species and 
individuals composing them, in their relations to one another and to 
their common environment which constitutes the main subject 
matter of ecology. 
I think there is no general fact connected with plant-distribution 
which has struck me personally with greater force than the very 
close similarity of the plant-associations living in similar habitats 
in different parts of the globe. With certain notable exceptions 
the vegetation of the river-side, of the sea-shore, of the alpine 
summits of the tropics possesses exactly the same characteristics 
as that of a similar locality at home in Western Europe. In spite 
of the enormous differences of climate, i.e., of temperature, rain¬ 
fall and air humidity, the compelling effect of the topographical 
factor in forcing, so to speak, the vegetation found in similar types 
of habitat along parallel lines, into parallel associations, is every¬ 
where seen. 
It is obvious that the study of such associations falls under 
two heads, corresponding to the two stages of procedure inevitable 
in natural science. First, the mainly descriptive stage. The plant- 
associations which are the objects of our study must first be 
characterised, enumerated, and described. The different species 
making up an association must be catalogued, their obvious relations 
described, the physical conditions under which they exist taken note 
of. The variations of each association must be recorded, as well 
as the transitions from one to another. This is the business of 
ecological survey. Without it, our knowledge, regarded merely from 
the descriptive standpoint, of the flora of a country or region of the 
earth, is entirely one-sided and inadequate. We cannot content 
ourselves merely with the knowledge that certain species of plants 
occur. We must know exactly how they occur, in what situations, 
associated with what other plants, before we can have a complete 
mental picture of the vegetation. Such knowledge is as theoretically 
justifiable and necessary as, and, in fact, is in every respect comparable 
with, that gained by a geological survey and the construction of 
geological maps. The philosophers tell us that the first object of 
that human curiosity which is the basis of all science is to “intuit,” 
i.e., to make a direct mental picture of, the contents of space and 
time. We cannot intuit the contents of space botanically without the 
method of ecological survey. In the old days, when vast tracts of the 
earth’s surface were still unexplored, when botanical collecting in a 
new country meant that the great majority of species brought home 
were still undescribed, it was natural that the collection and 
