1 90 STRUCTURE AND LIFE HISTORIES 



its roots deeper, if light is entirely cut off no chlorophyll 

 will form. A leaf, or the prothallus of ferns, is bilaterally 

 symmetrical because the environment is uniform on all 

 sides; the same organs have dorso-ventral differentiation 

 largely because the environment is unlike above and be- 

 low. The motility of sperms is an adjustment to water in 

 the environment. Thus variations in the environment 

 may result in different expressions of inheritance, just as 

 variations in inheritance would be followed by differences 

 in expression, even in an unchanging environment. In 

 order correctly to understand a plant nothing is more 

 necessary than to remember that its characteristics are 

 the result, not of its inheritance alone, nor of its environ- 

 ment only, hut of the interaction between the two. 



173. Struggle for Existence. — In Chapter XII atten- 

 tion was called to the fact that a moderate-sized fern pro- 

 duces each year about 50,000,000 spores. If each one of 

 these spores ultimately produced a mature fern-plant, and 

 if we allowed only i square foot of ''elbow room" for each 

 plant, the progeny of one parent only, in one season would 

 require at least 50,000,000 square feet, or nearly 1% 

 square miles. If each of these plants in turn, produced 

 50,000,000 offspring the next season, the descendants of 

 only one fern plant would, in only two years, cover the 

 stupendous area of over 83,000,000 square miles, or an 

 area equal to that of the North American Continent. 

 It has been calculated that a single plant of hedge mustard 

 may produce as many as 730,000 seeds. If each seed 

 developed another full-grown plant, and if the plants were 

 distributed 73 to each square meter, there would be enough 

 mustard plants to cover an area equal to 2,000 times 

 the dry surface of the earth. One may easily imagine 



