METHOD OF NATURAL HABITATS ; 307 
is designated the method of natural habitats. In contrast with this is the 
method of artificial habitats, in which the habitat itself is definitely modified, 
or a group of species actually transferred to a different habitat. Many prob- 
lems of vegetation can be attacked with greater success under control than 
in the field. “This is particularly true of competition, in which results can be 
obtained most readily by means of the method of control habitats, as carried 
on in the plant house. 
METHOD OF NATURAL HABITATS 
357. Natural experiments. Every family as well as every community 
constitutes an experiment in competition; the same statement necessarily 
holds for the larger groups, society, consocies, and formation, which are com- 
posed of families and communities. The last also make it possible to study 
competition in two typical instances, viz., in the family, where the individuals 
are of one kind, and in the community, where they belong to two or more 
different species. The community, moreover, is a product of invasion, and it 
furnishes material for the study of this function, as well as for that of aggre- 
gation and competition. Practically every formation shows some invasion, 
but as a rule stable formations contain so few invaders that they are rela- 
tively unimportant in this connection. Invasion is most active in transition 
areas and in mixed formations, whether produced by juxtaposition or by 
succession, and its study in these places vields by far the largest number of 
valuable results. 
As typical complete invasion, a succession is the best of all natural ex- 
periments in aggregation, migration, ecesis, and competition. This is espe- 
cially true of the initial stages in which changes in the number and position 
are most readily followed. The methods used in studying successions have 
been given elsewhere. In addition, it should be pointed out that one of the 
first tasks in taking up the ecological investigation of a region is to make a 
careful search for all new and denuded areas, as well as for those in which 
succession is taking place. The phenomena in these areas can not be ex- 
plained until the habitats and formations have been worked over critically, 
but the facts must be collected at the earliest possible moment, since the 
stages of the succession are constantly changing, while the stable formations 
are not. 
METHOD OF ARTIFICIAL HABITATS 
358. Modification of habitat. As the final factors in ecesis and competi- 
tion, water, light, and temperature control the grouping of plants into vege- 
tation. An efficient change in one of these, or in all of them, brings about a 
visible adjustment in the structure of the plant group concerned. Modifica- 
