226 THE FORMATION 
sary factor. Closed associations usually act as complete barriers, while open 
ones restrict invasion in direct proportion to the degree of occupation. To 
this fact may be traced a fundamental law of succession, viz., the number 
.of stages in a succession is determined largely by the increasing difficulty 
of invasion as the habitat becomes stabilized. Man and animals affect mi- 
gration directly, though not obviously, by the destruction of disseminules. 
They operate as a pronounced barrier to ecesis wherever they alter condi- 
tions in such a way as to make them unfavorable to invading species, or 
when, by direct action upon the latter, such as grazing, tramping, parasit- 
ism, etc., they turn the scale in the struggle for existence. The absence of 
insects adapted to insure fertilization is sometimes a serious barrier to the 
establishment of adventitious or introduced plants. The presence of para- 
sitic fungi, in so far as they destroy the seeds of plants, acts as an obstacle 
to migration,.and restricts or prevents ecesis in so far as the fungi destroy 
the invaders, or place them at a disadvantage in the struggle for existence. 
275. Influence of barriers. Physical barriers are typically permanent in 
character, while biological ones are either permanent or temporary, depend- 
ing upon the permanence of the formation and the constancy of the physical 
factors which determine it. A stable formation, such as a forest or meadow, 
which acts as a decided barrier to invasion from adjacent vegetation, may 
disappear completely, as a result of a landslide, flood, or burn, or through 
the activity of man, and may leave an area into which invaders crowd from 
every point. Often, without undergoing marked change, a formation which 
has presented conditions unfavorable to the ecesis of species of mesophytic 
character may, by reason of a temporary change in climate, become suffi- 
ciently modified to permit the invasion of.mesophytes. On the other hand, 
a meadow ceases to be a barrier to prairie xerophytes during a period of 
unusually dry years. A peculiar example of the modification of a barrier is 
afforded by the defoliation of aspen forests in the mountains as a result of 
which poophytes have been enabled to invade them. Nearly all xerophytic 
stretches of sand and gravel, dunes, blowouts, gravel slides, etc., and even 
prairies to a certain degree, exhibit a recurrent seasonal change in spring, 
as a result of which the hot, dry surface becomes sufficiently moist to per- 
mit the germination and growth of invaders, which are entirely barred out 
during the remainder of the year. In an absolute sense, no barrier is com- 
plete, since the coldest as well as the dryest portions of the earth’s surface 
are capable, at times at least, of supporting the lowest types of vegetation. 
Relatively, however, in connection with the natural spread of terrestrial 
plants, it is possible to distinguish partial barriers from complete ones. 
Such a distinction is of importance in the consideration of invasions from 
