INVASION 225 
this last case, however, they are as a rule recurrent. ‘Barriers may further- 
more be distinguished as complete or incomplete with respect to the 
thoroughness. with which they limit invasion. Finally, the consideration of 
this subject gains clearness if it be recognized that there are barriers to. 
migration as well as to ecesis, and if we distinguish barriers as physical or 
biological with reference to the character of the feature concerned. 
273. Physical barriers are those in which limitation is produced by 
some marked physiographic feature, such as the ocean or some other large 
body of water, large rivers, mountain ranges and deserts (including ice 
and snow fields). All of these are effective by virtue of their dominant 
physical factors ; hence they are barriers to the ecesis of species coming from 
very different habitats, but they act as conductors for species from similar 
vegetation, especially in the case of water currents. A body of water, repre- 
senting maximum water-content, is a barrier to mesophytic and xerophytic 
species, hut a conductor for hydrophytic ones; deserts set a limit to the 
spread of mesophytic and hydrophytic plants, while they offer conditions 
favorable to the invasion of xerophytes; and a high mountain range, be- 
cause of the reduction of temperature, restricts the extension of macrother- 
mal and mesothermal plants. A mountain range, unlike other physical 
barriers, is also an obstacle to migration, inasmuch as natural distributive 
agents rarely act through it or over it. 
274. Biological barriers include vegetation, man and animals, and plant 
parasites. The limiting effect of vegetation is exhibited in two ways. 
In the first place, a formation acts as a barrier to the ecesis of species in- 
vading it from the formations of another type, on account of the physical 
differences of the habitats. Whether such a barrier be complete or partial 
will depend upon the degree of dissimilarity existing between the forma- 
tions. Hylophytes are unable to invade a prairie, though open thicket plants 
may do so to a certain degree. In the same way, a forest formation on 
account of its diffuse light is a barrier to poophytes; and a swamp, because 
of the amount and character of the water-content, sets a limit to both 
hylophytes and poophytes. Formations, such as forests, thickets, etc., 
sometimes act also as direct obstacles to migration, as in the case of tumble- 
weeds and other anemochores, clitochores, etc. A marked effect of vege- 
tation in decreasing invasion arises from the closed association typical of 
stable formations and of social exclusive species. In these, the occupation 
is so thorough and the struggle for existence so intense that the invaders, 
though fitted to grow under the physical factors present, are unable to com- 
pete with the species in possession for the requisite amount of some neces- 
