NATURAL SELECTION AND EVOLUTION 567 



by insects, unfavorable weather or other agencies while they are 

 mere embryos within the seed. Many are destroyed in the 

 seedling stage and others at later stages in life. 



In seeding many of the cultivated crops we reckon with the fact 

 that many of the individuals fail to complete their development. 

 In ten pounds of Red Clover seed, the amount commonly sown 

 per acre, there are about two million seeds. If each seed pro- 

 duced a plant large enough to occupy one half of a square foot of 

 space, there would be enough plants to cover more than twenty 

 acres, but we are satisfied with a good stand on the one acre. 

 This means that the number of plants reaching maturity are few 

 in comparison with the number of seeds sown. In some of the 

 seeds the embryos are killed while they are developing or during 

 storage. Others are so slow in germinating that the seedlings 

 are killed by the drought or heat of summer. Some plants are 

 killed by insects, Fungi, and Bacteria; others are crowded out 

 by the stronger Clover plants or by weeds. In seeding Timothy 

 and Blue Grass, even more allowance is made for the destruction 

 of individuals than in the case of the Clovers. If all the kernels 

 of Wheat sown produced plants averaging three heads per plant 

 and twenty- five kernels per head, from each bushel of Wheat 

 sown there should be a yield of seventy-five bushels, but one third 

 of this amount is a good yield for each bushel of seed sown. 



In nature, as among weeds and wild plants in general where 

 man does not interfere, the mortality is exceedingly great. The 

 Green Foxtail, a common weed in truck patches, produces from 

 500 to 2000 seeds per plant. In a few years there would be no 

 room on the land for any other plants if all the seeds of the 

 Foxtail developed mature plants. The same is true with most 

 weeds. Among trees, such as Maples, Oaks, Willows, and Poplars, 

 thousands of seeds are commonly produced, but only a few trees 

 develop therefrom. 



Through the struggle for existence, some kinds of plants are 

 often entirely replaced by others. If a plot of ground is left 

 uncultivated, the first plants that come are usually replaced later 

 by other kinds. In many parts of the United States Timothy is 

 soon replaced by Redtop or other grasses and by weeds, and the 

 meadows have to be plowed and set again to Timothy. Many of 

 our common weeds have been replaced by weeds from foreign 

 countries. 



