394 ELEMENTARY ZOOLOGY. 



actual course of evolution in any given instance. Why 

 has development taken the course it has? 



Theoretically there might be an inherent tendency in 

 living matter at the beginning to vary or evolve in a cer- 

 tain direction. Biologists are not agreed upon the exist- 

 ence of such a tendency. On the other hand, the environ- 

 ment meaning the total external conditions of the life of 

 organisms may guide or direct the course of develop- 

 ment. Any guidance must come from the one or the 

 other of these sources. 



The environment may act in either of two ways to 

 mold evolution. We have seen in the first place that the 

 environment does act directly to produce changes in 

 organisms. If these changes can be inherited, this will 

 be a most important means of guiding evolution. It is 

 not certain, however, that these direct effects of the environ- 

 ment on the body of an organism can be transmitted. 



Whether this is true or not, there is another effect of 

 the evironment which has certainly had a great influence 

 on the course of evolution. The principle was discovered 

 by Darwin and by him called "natural selection." It 

 refers to the fact that the struggle for existence among 

 organisms is so severe that some will inevitably be elimi- 

 nated. We have seen that this is true among all species. 

 A hundred are born of two parents : only two of these, on 

 an average, will be preserved. Which will be successful? 

 In the long run, those will be preserved which are best 

 adjusted to the conditions of the life which they meet. 

 This fact is independent of how they came to be adjusted. 

 This is known as the " survival of the fittest. " " Fittest ' ' 

 merely means those best adapted to live and reproduce in the 

 environment encountered. This process of natural selec- 

 tion does not cause variations, but it may make use of any 



