CHAPTER XXXVII 

 PLANT DISTRIBUTION 



The Facts of Distribution. — Almost everyone has 

 travelled far enough to realize that the assemblages of 

 plants and animals in different places are more or less 

 different. The chief underlying causes of this fact be- 

 come more obvious when distant areas are considered, as, 

 for example, North America, South America, Asia, and 

 Africa. If one were to find an area in each of these 

 continents in which the conditions of soil and climate 

 are so nearly the same that the same plants can be culti- 

 vated equally well in all, he would nevertheless find that 

 the plants native in each area differ from those in all 

 the others, but that there exists a large measure of super- 

 ficial resemblance in the general appearance or aspect of 

 the assemblages of plants. When the individual plants 

 are considered it is found that the difference is much 

 greater than the general appearance in the mass would 

 suggest. From these observations we may deduce two 

 conclusions: first that mere distance has something to 

 do with the distribution of plants and second that 

 similarity of habitat has a good deal to do with similar- 

 ity of appearance in the mass. For example, if one of 

 the supposed areas is a forested one it is quite certain 

 that all the others will be also. 



The General Causes of Distribution are already 

 familiar to the reader for he knows that organisms under- 

 go mutations or heritable variations and that they are 

 able to spread themselves over considerable areas either 

 because they themselves are motile or because their seeds 

 or spores can be carried by wind, water, or animals. 



449 



