So6 ZOOLOGY 



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 or not the environment can act to produce inher- 

 itable changes in organisms, 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 nor how small 

 in amount, provided they can be inherited; that is to say, if 

 only these variations are really mutations. In so far as it acts, 

 its result is to guide evolution into suitable adaptation to the 

 conditions that are able in any degree to influence life. Nature 

 undoubtedly can exercise at least a veto power on the direction 

 of evolution. Darwin's doctrine of selection has been much 

 criticized in recent years. It is, however, the only theory for the 

 guidance of evolution for which there is any definite evidence. 



In brief, the origin of heritable variations is in the germ 

 plasm, produced certainly by new combinations of germ plasm, 

 and conceivably by the direct or indirect influence of the environ- 

 ment upon the germ plasm. And the guidance of these varia- 

 tions into permanent evolutionary results is secured by the 

 selective power of the environment. 



497. Evolution and Man. — Many people are deterred from 

 accepting the general theory of evolution because of unwilling- 



