500 RELATION TO ENVIRONMENT. 



966. Opportunities for migration of plants between conti- 

 nents. Continuity of land areas provides opportunity for migra- 

 tion of plants. Discontinuity of land sets up a barrier to plant 

 migration. Where comparatively large land areas are separated 

 by comparatively narrow bodies of water, their proximity affords 

 greater possibility of the plants passing the barrier, either by 

 floating of plant parts, or seeds, or by seeds being transported 

 by wind or animals. Looking at the map of the northern hemi- 

 sphere in the region of the arctic circle, it is seen that with north 

 British America, Alaska, Siberia, Russia, Norway and Sweden, 

 the British Isles, Iceland, Greenland, there is a nearly continuous 

 area of land encircling the globe just below the pole. Plant 

 migration from one continent to another here is not so difficult as 

 farther south. Before the glacial periods it was much warmer 

 in the region of the arctic circle, as we learn from the fossil 

 remains of tropical plants found there. In that remote age 

 (tertiary times) it is likely there was communication at this point 

 between plants of the northern continents. But it is very differ- 

 ent in the southern hemisphere. The continents of Africa and 

 South America are far separated, and migration of plants be- 

 tween the two continents is well-nigh impossible, and probably 

 has occurred only in exceptional cases. These relations of the 

 continents help to explain why it is that there are so many resem- 

 blances between the flora on the continents of Europe, Asia, and 

 North America, while there is little resemblance between the 

 flora on the continents of Africa and South America. 



II. Life Regions, Zones, and Areas. 



967. Lines of plant migration. Plant migration takes place 

 along lines of least resistance. Like other questions in plant 

 distribution this one is complex. In general it may be said that 

 the lines of least resistance are those of like temperature areas, 

 and of areas with like moisture content. Plants move more 

 freely over belts the temperature and moisture conditions of 

 which are favorable for their growth and reproduction. Evi- 



