138 DISSEMINATION OF SEEDS. 



Such facts as these have led to many discussions and 

 the belief at present is that, during the Glacial Period 

 which is plainly marked by bowlders and scored rocks in 

 various parts of Europe and America, as the southern 

 regions became fitted for the plants of the north these 

 latter took the place of the original inhabitants of the 

 warm region ; those of these warm regions travelled 

 farther south so that the soil of temperate regions became 

 covered with plants from arctic regions and that of trop- 

 ical regions with plants from the temperate regions. As 

 this period was succeeded by one of increasing warmth, 

 the reverse process was carried on, one result being that 

 at its close there were found on the tops of the mountains 

 the plants originally found in the cooler northern regions. 



We can also thus understand why European and Amer- 

 ican plants are so closely allied ; for then, as now, the 

 northern lands were nearly united and the same plants 

 probably flourished in all parts of the polar regions ; as 

 the cold period moved from north to south on certain me- 

 ridians, so the plants of northern regions moved south and, 

 with the increase in the distance between two meridians 

 as they extend towards the equator, became scattered over 

 broad tracts of country. 



DISSEMINATION BY BIRDS. 



Birds must be mentioned among the important agents 

 in the dispersal of seeds over a broad expanse of land or 

 of water. Many seeds are thus dispersed because their 

 fleshy pericarps are used for food. Birds often carry 

 cherries to some convenient place for eating the pericarp. 

 The stone, not being useful for food, is dropped and from 

 it there may spring up cherry trees far away from the 

 tree which produced the fruit. In some places these are 



