26 MAN AX ADAPTIVE MECHANISM 



the conditions governing the struggle vary, and the 

 evolved mechanisms are correspondingly distinctive. 

 In the waste places of the earth the struggle is more 

 particularly with the climate against the severity 

 of heat, cold and drouth, and against a scarcity of 

 food. Here may be seen such devices as are found 

 among the plants of the desert gaunt, geometrical 

 forms covered with spikes instead of leaves, to prevent 

 the evaporation of moisture, and with roots which 

 are many times the size of the bush above, that they 

 may make vast ramifications into the earth below in 

 a search for water. Again, in more verdant spots, 

 where food is plentiful, but many individuals are 

 seeking it, the fight for possession is between species 

 as well as within each species. Here the plant, sup- 

 plied with abundant moisture but surrounded by a 

 thicket of other equally eager plants, must evolve a 

 device for scattering its seeds abroad. Hence come 

 the various contrivances of fruit and flowers by which 

 animals are enticed to devour the seed and eject it ; 

 or by which the seeds are easily disseminated by the 

 wind balloon-like pods in which the seeds are encased, 

 easily rolled along by the wind; hooks or spikes by 

 means of which the seeds adhere to the hides of beasts ; 

 or elegant plumage like that of the dandelion, by means 

 of which the seeds are lightly wafted by the breeze. 

 Each variation, as it occurs sporadically, is adjudged 

 by the environment, condemned and suppressed if 

 harmful ; approved, perpetuated and perfected if 

 advantageous. 



The same end is attained by a variety of means, 

 each organism seeming to have hit upon a combina- 



