1911} COWLES—VEGETATIVE CYCLES 167 
give way to a comparatively small number of relatively permanent 
associations. 
In 1888 TREvB, whose recent premature decease we so keenly 
regret, began the study of the new vegetation of Krakatoa (12), 
thus inaugurating one of the most fruitful lines of investigation in 
dynamic plant geography. In 1891 WARMING, to whom more than 
to any other we owe the present large place occupied by ecological 
plant geography, published the first of his developmental studies 
of Danish dune vegetation (13). This was followed by a similar 
treatment of the Rhéne delta by FLaHAvLT and Compres (14), and 
of the North German heath by GRAEBNER (16), and also by WARM- 
_ING’s Plantesamfund (15), the original Danish edition of his well 
known Plant geography, in which there is much material of dynamic 
import, together with the formulation of a number of “laws of 
succession.”” In 1896 MEIGEN (17) made a systematic study of 
Succession, somewhat along the lines previously followed by Hutt, 
and he showed that there is a final tendency toward equilibrium. 
This brings us to the period in which dynamic plant geography 
was taken up actively in this country, and here our historical résumé 
may well give place to the main topic of this paper. 
3. The delimitation of successional factors : 7 
No systematic attempt has been made hitherto to group in an 
analytic manner the phenomena of succession from the standpoint 
of their causation. WARMING (15) made a great advance toward this 
end by gathering together the known records of vegetative change 
°F succession ;. he noted that vegetative change is particularly evi- 
dent on new soil (as along sandy shores, and in marshes, on lava, on 
‘andslip soil and talus, and on burned and fallow land). He sum- 
Marizes his studies by giving six laws appertaining to succession. 
CLEMENTS (21) attempted to distinguish between primary and 
secondary successions, the former being those on newly formed 
Soils, and the latter those on denuded soils. This classification 
seems not to be of fundamental value, since it separates such closely 
related phenomena as those of erosion and deposition, and places 
together such unlike things as human agencies and the subsidence 
ot land, CLeMEnTs, like WARMING, gives a summary of results in 
the form of laws. 
