104 THE BIOLOGY OF A PLANT. 



process in the history of each individual plant. And there is 

 every reason to believe that this is typical of the history of each 

 species of plant or animal in the past. We know that environ- 

 ments change, and that to a certain extent organisms change 

 correspondingly, provided the change of environment be not too 

 sudden or extreme. The environment of Pteris changes periodi- 

 cally with the regular alternation of summer and winter, and the 

 plant also undergoes a corresponding periodic change of struc- 

 ture in order to maintain its adaptation to the environment. 

 During the summer the aerial part is fully developed, and, as a 

 result of its activity, starch is accumulated in the rhizome. At 

 the approach of winter the aerial part dies, and the plant is re- 

 duced to the underground part safely buried in the soil. Dur- 

 ing the winter and spring the starch is gradually consumed, and 

 the aerial part is put forth again as the aerial environment be- 

 comes once more favorable to it. The organism, therefore, 

 possesses a certain plasticity which enables it to adapt itself to 

 gradually changing conditions of the environment. 



A little consideration will show that every function or action of living 

 things may be regarded as contributing to the same great end; viz., har- 

 mony with the environment ; and from this point of view life itself has 

 been denned as "the continuous adjustment of internal relations to ex- 

 ternal relations.' 1 ' 1 * 



Now there is good reason to believe that as environments 

 have gradually undergone extensive changes in the past, organ- 

 isms have gradually undergone corresponding changes of struc- 

 ture. Those which have become so modified as to be most 

 perfectly adapted to the changed environment, have tended to 

 survive and leave similarly adapted descendants. Those which 

 have been less perfectly adapted have tended to die out through 

 lack of fitness for the environment ; and by this process- 

 called by Spencer the " Survival of the Fittest " and by Darwin 

 *' Natural Selection"- the remarkable adaptations everywhere 

 met with have been gradually worked out. 



Nutrition. The fern does work. In pushing its stem through 

 the soil, in lifting its leaves into the air, in moving food-matters 

 from point to point, in building new tissue, in the process of re- 



* Spencer, Principles of Riology , Vol. I., p. 80. N. Y., Appleton, 1881. 



