496 ZOOLOGY 



from evening up and thus destroying their variations by inter- 

 crossing, it still remains to find the factors that have determined 

 and guided the actual course of evolution in any given instance. 

 Why has development taken the course it has ? 



Theoretically, as we have seen, there might be an inherent 

 tendency in living matter at the beginning to vary or evolve in 

 a certain 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 development. 

 Any guidance must come from the one or the other of these 

 sources, internal or external. 



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 specific 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 environment on the body of an organ- 

 ism can be transmitted to the germ cells. External conditions 

 may likewise act directly upon the germ plasm so as to produce 

 changes in its nature. Doubtless such changes would perma- 

 nently affect future generations. 



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

 environment 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 eliminated. 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 ad- 

 justed. This is known as the "survival of the fittest." "Fit- 

 test" 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 

 variations that arise, no matter what their source, provided they 



