NATURAL SELECTION 135 



but the group as a whole has continued to carry on its 

 lowly functions in its relatively simple way. On the 

 other hand, the higher forms of life exhibit innumerable 

 structural modifications, which adapt them to all sorts 

 of special conditions. The effects of natural selection 

 are in. proportion to the necessities of the organism, in 

 relation to the environment. If no change is advan- 

 tageous, selection itself will destroy all variations, and 

 hold the creature true to type. Thus it can just as well 

 prevent evolution as cause it. But when conditions are 

 changing, or new adaptations permit entrance into new 

 fields of opportunity, selection is a powerful factor, pro- 

 vided that the necessary heritable variations occur. In 

 the absence of such variations there may be no "fittest" 

 to survive, and the species becomes extinct. 



10. Regarding the matter quite broadly, there can Complexity 

 be no doubt that the beauty and variety of living things 



has been brought about through the agency of selection, result of 

 The rocks are full of fossil types which have perished ; 

 every species has had to endure the test of fitness. In 

 a sense, the motive force of evolution has been environ- 

 mental change, compelling adaptations as the price of. 

 existence. Without periods of heat and cold, moisture 

 and dryness, without a world presenting all sorts of 

 different conditions, evolution could not have taken 

 place. Man is the outcome of innumerable trials., in- 

 numerable adjustments. As life has become more 

 varied, each type has become part of the environment 

 of others, as their prey, or their enemy, or as occupy- 

 ing space they would possess. Thus the complexity of 

 adjustment has increased by a principle of acceleration 

 growing out of itself, like progress in human society. 

 Whichever way we regard the matter, we can only come 

 back to the great realities. What we see represents the 



