PLANT BREEDING AND EVOLUTION 209 



The ideas concerning selection through struggle for existence, 

 emphasized in the above extract from the " Origin of Species," 

 can be most easily understood from concrete illustrations. The 

 writer once made an estimate of the number of seeds borne by 

 the common dandelion in a single season. Rather large, healthy 

 plants produced on the average about ten fruiting scapes, and 

 each seed ball on a scape averaged from 160 to 260 seeds. This 

 would mean an average of from 1600 to 2600 seeds borne on 

 each dandelion plant in a single season. The Russian thistle has 

 been estimated to produce from 20,000 to 200,000 seeds .011 a 

 single plant. If all of these seeds gained a foothold and grew 

 into offspring, it can readily be understood that the earth might 

 be entirely populated with dandelions or Russian thistles in a 

 comparatively few years, since both plants are well adapted for 

 distributing their seeds. It must be remembered, however, that 

 the plants with which the dandelion and Russian thistle have to 

 compete for soil space, air, and light are also producing seeds 

 and offspring at corresponding rates, and that some of their com- 

 petitors, like the grasses, have vegetative means for spreading 

 which are quite as effective as seed production. It is conceivable, 

 therefore, as Darwin has said, that in the fierce competition 

 among plants, due to overproduction, only those individuals 

 would survive in a given environment which possessed structural 

 and physiological characteristics best adapted to the conditions 

 of that environment. The remainder would die, and the best- 

 adapted plants would live to perpetuate the race, just as the 

 particular races of cultivated plants now in existence are sur- 

 vivors from plants formerly selected by man. In this manner 

 any given race would gradually improve, and so evolution would 

 take place. This selective principle is also easily understood if 

 we contemplate extreme conditions such as obtain in deserts and 

 in lakes or ponds. The vegetation of these two habitats is very 

 distinct, for the reason that extreme drought, on the one hand, 

 and excessive water supply, on the other, destroy all seeds, seed- 

 lings, or mature plants which are not definitely adapted to a 

 xerophytic or a hydrophytic habitat. Each year seeds from other 

 habitats are distributed over deserts and lakes, but only those 



