58 INVASION 



invasion. Finally, the consideration of this subject gains 

 clearness, if it be recognized that there are barriers to migra- 

 tion as well as to ecesis, and if we distinguish barriers as 

 physical or biological with reference to the character of the 

 feature concerned. 



Physical barriers (dbices physicae) 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, representing maximum water-content, is a barrier to 

 mesophytic and xerophytic species, but a conductor for 

 hydrophytic ones : deserts set a limit to the spread of meso- 

 phytic and hydrophytic plants, while they offer conditions 

 favorable to the invasion of xerophytes ; and a high mountain 

 range, because of the reduction of temperature, restricts the 

 extension of macrothermal and mesothermal plants. A 

 mountain range, unlike other physical barriers, is also an 

 obstacle to migration, in as much as natural distributive 

 agents rarely act through it or over it. 



Biological barriers (obices bioticae) include vegeta- 

 tion, 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 

 invading 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 formations. Hylo- 

 phytes 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 char- 

 acter 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 



