Dispersion of Fruits and Seeds. 209 



and raw materials from the soil. Or, if the parent were a 

 perennial, it would overshadow and starve the seedlings. 

 Even with some means of seed dispersion, there is apt to 

 be greater competition between individuals of the same 

 species than between those of different species, because 

 plants of the same species require the same proportions of 

 the different soil constituents, and, having the same habit 

 of growth, crowd each other more than the same number 

 of individuals of different species distributed over the same 

 area would be apt to do. One need only compare the 

 number of seedlings of a particular species appearing above 

 the soil in a given area with the number of those which 

 actually reach maturity in the same area to be convinced 

 of the struggle which is taking place among them, with 

 fatal results except to the relatively few individuals which 

 are stronger and more rapid growers than their fellows. 

 Any devices which aid in dispersing the seed help to 

 ameliorate these conditions. 



144. Migration by Seeds. — The dispersion of seeds and 

 other reproductive bodies has been of vital importance to 

 plants in another way : The climate of the earth has been 

 undergoing profound changes through the long geological 

 periods. In some places a temperate, or even a warm, 

 climate has given way to a frigid one ; and then after a 

 long time the temperate climate has returned. By means 

 of fruits and seeds capable of dispersion, plants have been 

 able to recede before the advancing cold, when without 

 this power they would have become extinct. And finally 

 when the warmer climate returned, they have been able to 

 follow the retreating ice back toward the arctic circle. 



In the same way they have taken possession of land 

 which has risen on the borders of continents, or far out in 

 the oceans in the form of volcanic or coral islands, and of 



