PLANT-FORMATIONS. 897 



fortunate, but having been once introduced into the science, it must remain, and it 

 is only necessary to point out that the communities united in a formation do not 

 always exhibit any obvious stratification Thus, for instance, in many tropical 

 forests (see fig. 420, p. 741) communities are interpolated which belong to the most 

 widely different types, and exhibit all possible grades in respect of the height to 

 which their component plants grow. These communities occupy sometimes only a 

 restricted area, sometimes a considerable expanse either down on the ground or 

 midway between the ground and the tops of the trees; and, moreover, in all such 

 formations there is always a sprinkhng of climbing-plants and epiphytes, which 

 make it quite impossible to discover distinct strata. In many other cases, it is true, 

 the communities constituting a formation are in obvious strata. If we may compare 

 the plant-formation to a building, the communities may be said to form the stories 

 risiag one above the other. Sometimes only two communities are superimposed on 

 one another, sometimes it is possible to distinguish three or more strata or stories. 

 There are formations in which each story belongs to a different class of community, 

 but others exist also where two or three of the stories are of the same class, as, for 

 instance, where several scrubs rise one above another, or where two forests are 

 united, so that the crown of one species of tree forms an upper tier, and that of 

 another species a lower tier. 



The names of the different plant-formations should be chosen with regard to the 

 community which forms the roof of the entire edifice of plants in each case, and 

 which, therefore, projects above, and, in a sense, prevails over all the other com- 

 munities. 



At the beginning of this chapter stress was laid on the fact that every region 

 receives a characteristic impress from the nature of the plant-communities inhabiting 

 it, and that a knowledge of the latter is consequently of great importance in descrip- 

 tive geography. The remark has also been made more than once that the particular 

 conditions of soil and climate in a locality find expression in plant-communities, as 

 it must be presumed that the species characteristic of each community can only grow 

 in masses at places where the composition of the soil and the conditions of illumina- 

 tion, temperature, and humidity are in harmony with the specific organization as a 

 whole. But if the local conditions of the ground and climate are reflected in the 

 plant-communities, it is equally the case that the distribution of the plant-communi- 

 ties and formations constitutes an important and perhaps the only available basis 

 for a division of the earth into natural floral areas. We proceed on the principle 

 that every district possessing a series of plant-communities which are pectdiar to 

 itself ia to be treated as a floral area, and that a limit to such area occurs at every 

 place where the characteristic communities of a particular floral area are threatened 

 with destruction, and, therefore, encounter the natural boundary of their range, 

 where other communities better adapted to the altered external conditions make 

 their appearance, and where there is consequently a change in the aspect of the 

 whole landscape. This also supplies the scheme for a scientific geography of plants. 



Unfortunately we are still far from possessing any such science. We have only a 

 Vol. ii 107 



