METHOD OF ARTIFICIAL HABITATS 309 
Quadrat, transact, and migration circle are equally valuable for ecesis and 
competition. The quadrat is best adapted to work in a homogeneous area, 
while the transect is suited to a heterogeneous one characterized by zones, 
societies, or communities. It is an advantage to replace the denuded tran- 
sect by a series of denuded quadrats, one for each zone or society, when the 
transect would be too long for convenience. The denuded migration circle 
is invaluable for aggregation and ecesis, since it makes possible the study 
of migration as a distinct function. A series of denuded quadrats, con- 
sisting of one or more in the different stages of a succession, furnishes 
important evidence concerning the development of each stage. By far the 
best method, however, for making a comparative study of the stages of a 
succession is the quadrat sequence. A quadrat is denuded each year, thus 
yielding a complete sequence of miniature stages through the whole course 
of succession. This method is especially valuable when a succession is 
teptesented by a single example, and there is no opportunity of reconstruct- 
ing it by the comparison of various stages. ‘A quadrat sequence is naturally 
of the greatest value if begun at the time when the first invaders appear. 
360. Modification of the formation by transfer. The study of partial 
and intermittent invasion into an established vegetation is made through 
the transfer of a species or group of species by means of seeding or plant- 
ing. The process differs in no way from that described for experimental 
evolution, except in so far that an endeavor is made to establish a family or 
a community, and not merely a few individuals. Transfer makes possible 
the critical investigation of ecesis under conditions of intense competition, 
as well as the study of aggregation and the origin of plant groups under these 
conditions. Perhaps its greatest value is in the experimental study of al- 
ternation and zonation, especially the former. It is practically impossible 
to determine whether alternation, especially when corresponsive, is due to 
physical or historical causes, i. e., migration and competition, except by 
means of the reciprocal transfer of the species concerned. 
Field cultures for the careful study of ecesis and competition are made by 
transferring seeds or plants to new or denuded soils. This is practically 
a combination of the methods of modification and transfer. It has a unique 
value in making it possible to initiate artificial successions of almost any 
character that is desired, and to carry them out with the reactions more or 
less under control. This opens up an extremely important field of ex- 
perimental inquiry, which promises to put the study of succession upon a 
much more exact basis. Competition cultures in the field are not essentially 
different from those under control, and they will be considered under the 
next method. 
