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A. G. Tansley. 
the wide prevalence of the conditions under which they are able to 
exist. The same explanation holds in the case of the most widely 
spread genera of higher plants. Water plants are perhaps the 
most striking example. The conditions of life in water are far 
more uniform throughout the world than the conditions of life on 
land, while the ease with which aquatic plants are propagated 
vegetatively is well known, and these no doubt are the deter¬ 
mining factors in the world-wide distribution of so many of the 
higher water plants. 
Through these considerations we are naturally brought from 
the contemplation of what we may call geographical aggregates to 
those other aggregates which we may distinguish as topographical r 
and with which ecology in the sense we have defined, is mainly 
concerned. 
In any region of the earth, in any country, in any limited area, 
we find the flora—composed as a whole partly of members of the 
families characteristic of the region, and partly of the members of 
cosmopolitan families — naturally falling into more or less definite 
“plant-associations’* according to habitat. We find the plants of 
the mountain differing from those of the valley, the plants of 
marshy land differing from those of the dry plain, the plants of the 
coast differing from those of the interior, the plants of the sand- 
dunes differing from those of the salt-marshes. These differences 
are due to differences in the nature of the soil dependent largely 
upon the geological formations underlying the various areas; upon 
differences of water-content of the soil dependent partly upon the 
nature of the soil and partly upon the neighbourhood of large 
permanent bodies of water; upon differences of rainfall and of the 
humidity and movement of the air; upon differences of the salinity 
of the soil; and upon differences of temperature and illumination. 
The morphological and physiological characters of the plants 
which make up the different associations found in these different 
environments are, as is well known, the result of adaptation to the 
conditions of life. Broadly speaking it may be said that only those 
species or individuals which possess, or are capable of developing 
under the appropriate stimulus, the special characters fitting them 
to exist under the given environmental conditions, are able to 
occupy and maintain themselves in the areas subjected to those 
conditions. The gross morphological features thus originating 
under the stress of a given set of particular conditions are often 
common to a number of different species and thus give to the 
association a definite physiognomy. 
