BARRIERS TO UNIVERSAL DISTRIBUTION 453 



particular species that make up the plant assemblages, 

 while flora applies to the aggregate of species without 

 reference to the vegetation types which they may com- 

 bine to produce. 



The Barriers to Universal Distribution. — In the 

 preceding topics the conditions positively concerned in 

 diversifying the assemblages of plants and animals in 

 different regions of the world have been emphasized and 

 only bare mention made of certain negative factors of 

 extreme importance. One may express the idea in this 

 way: when plants and animals evolve into new species 

 these would spread to all parts of the earth unless there 

 were some sort of barrier to stop them or the conditions 

 of the environment were unsuited to their habits of life. 

 To put this positively one may say that in the absence 

 of barriers of any kind all species of plants would ulti- 

 mately spread all over the habitable world but would be 

 restricted to those particular local areas or habitats which 

 would be suitable to each. As a result of this there would 

 be much the same sort of vegetation types as now exists 

 but the species in each would be the same all the world 

 over. On the other hand, if all plants were suited to the 

 same environmental conditions but the present barriers 

 still existed, there would be a uniform vegetation within 

 each barrier limit Except of course for the fact that the 

 same barriers do not apply to all species), but the differ- 

 ences in the actual species now present in each great 

 division of the world would remain just as they are. 

 From considerations of this sort it is obvious that barriers 

 play an important role in determining the Geographical 

 Distribution of species and it will be pertinent to inquire 

 into the nature of barriers and to consider how they 

 operate. 



Tangible Barriers are literally those which can be 

 touched but the term is used practically for all those of 

 a material and non-living character. To any animal 

 which can walk, but cannot fly or swim, water consti- 



