472 GLEASON: THE PLANT ASSOCIATION 
Since migration requires time, uniformity of any species is an 
expression (to be considered in comparison with its mobility) of 
the length of time it has been in the association, and general uni- ° 
formity of an association is regularly correlated with its age. As 
illustration, the uniformity of a forest may be compared with the 
lack of uniformity in the clearing association which develops soon 
after lumbering. 
18. Excessive seed production is almost universal among plants, 
and leads to a great competition for the environment. The 
number of individuals of a species is therefore an expression of the 
degree of its adaptation to the environment. In the ‘rapid de- 
velopment of an association over a considerable area of ground, 
as a forest clearing, the most mobile species is naturally represented 
at first by the greatest number of individuals, irrespective of its 
adaptation, but as soon as the ground is occupied competition 
restricts it to its proper proportion. 
19. Within an association, minor variations in environment 
may affect certain susceptible species and not others, and as a 
result produce minor deviations from the usual uniformity. For 
example, in an area of woodland in central Illinois, of otherwise 
uniform structure, a shallow valley is occupied every spring by 
large numbers of Floerkea proserpinacoides. In this case the 
controlling feature seems to be the greater amount of water in the 
surface soil during the spring months. For other plants at the 
same season, and for all plants during the summer and autumn, 
this feature seems to be without effect. 
Recent introduction or slow migration of a species may also 
produce colonies which interrupt the general uniformity. An 
instance of the former case has been described (7, p. 520, 521) in the 
migration of introduced species into the aspen association of 
northern Michigan. Colonies of the second type are generally 
caused by the vegetative reproduction of the species, and the 
resulting compact groups of individuals have been termed families 
by Clements (2, p. 203). 
Among the larger, longer-lived, dominant species, accidents of 
immigration, such as proximity or a good seed year for one species, 
may cause the development of minor groups within the associ- 
ation, characterized by a few or only one of its usual dominants. 
