A Struggle for Existence 155 



plants has had to await the feeding activities of bees and 

 butterflies. The appearance of autumn coloring results from 

 the relative drying of the soil. Many mammals change the 

 texture and the pigment of their fur as the weather becomes 

 colder. Worms and larger creatures burrow deeper into the 

 earth. The nestlings have already learned to fly and with 

 the surviving parents prepare to leave for Florida and points 

 south. Each detail in all of these processes is in a way an 

 adaptation to life. And each adaptive act or structure is 

 limited in its effectiveness by conflicting processes and struc- 

 tures on the part of other living things. 



A Struggle for Existence 



There are always more mouths to feed than can be sup- 

 plied by the available food. More blades of grass always push 

 above the soil than can possibly be supplied by the moisture 

 and salts available, to say nothing of standing room. The 

 struggle, however, is for the most part the more immediate 

 one between the individual and changing moisture, heat and 

 light, and the presence of enemies large and small of other 

 species. A change in temperature is in most cases, for a 

 given plant, a more immediate problem that the need for 

 additional food. Fluctuations in external conditions, in- 

 cluding the numbers and varieties of other plants and ani- 

 mals, demand continuous adjustment. A plant may con- 

 quer the wind by yielding gracefully to its fury or may 

 resist it by building firm buttresses. All life is struggle from 

 the very nature of things, although the struggle is not for the 

 most part of a kind suggested by our own experience with 

 competitive conflicts. 



Darwin laid great emphasis upon the fact of individual 

 variations; and it is indeed true that no two individuals are 

 exactly alike. This means of course that at any given mo- 

 ment one individual may be more advantageously situated 

 than another. Further than this, however, is the fact that 

 no individual is exactly the same from moment to moment. 



