468 Plant Physiology 



regarded by Darwin as of less importance in evolutionary 

 progress (see section 287). 



In most of the present-day discussions respecting evo- 

 lution, natural selection is recognized as a potent force ; 

 but great diversity of opinion prevails with regard to the 

 magnitude of the variations by means of which progress in 

 selection is maintained. 



284. Rate of increase. - - A study of the theoretical 

 rate of increase of many organisms involves numbers which 

 are not easily grasped. A single tobacco plant may 

 produce from 500,000 to 1,000,000 seed. At the minimum 

 production mentioned the second year there would be 

 250 billion seed, and the product the third year would 

 be expressed by eighteen figures. At the rate of one seed 

 per square foot, this number would plant the surface of 

 the earth several hundred times over. A vigorous 

 specimen of the common dandelion under observation 

 produced in a season about thirty flower-heads, each 

 averaging about 300 seed, or 9000 seed for the season. 

 In this case, there would result at the end of the fourth 

 season 6,561,000,000,000 seed. 1 



Assuming the capacity of an organism to vary, the power 

 of the environment to suppress and exterminate unfitness 

 makes it a very strong factor in determining the nature of 



1 One instance from the animal side may be cited (adapted from 

 Jordan and Kellogg, "Evolution and Animal Life," p. 59), that of the 

 quinnat salmon of the Columbia River. This is a prolific fish whose 

 eggs and young are poorly protected and consequently devoured by 

 numerous greedy enemies. A female will ascend the river when four 

 years old and deposit about 4000 eggs, subsequently dying. Allowing 

 for 50 per cent of males and the normal period of maturity of females, 

 only five generations would be required, should all individuals survive, 

 for this fish to occupy far more than the volume of the sea. 



