474 ELEMENTARY BIOLOGY 



made on the supposition that by letting disease kill whom it 

 will, we shall at last have a population that is immune to all 

 diseases. The same people would do nothing to mitigate what 

 they call " the struggle for existence," for they believe that 

 this struggle is necessary to bring out the best qualities of 

 the race and to prevent the multiplication of the unfit. 



With the discoveries of recent years regarding the facts of 

 heredity, our whole view of evolution has undergone important 

 changes. For one thing, " struggle for existence " no longer 

 suggests the fierce competition between individuals of a species 

 that it formerly suggested. In the second place, the survival 

 of the fittest can be seen to add nothing nezv to the composi- 

 tion of a line of plants or animals. In the third place, the 

 characters that distinguish one race or variety from another 

 have not necessarily anything to do with being better fitted 

 to live. And, finally, we may think of the progressive changes 

 in species as resulting from the successive modification of the 

 germ plasm, with the elimination of those resulting forms that 

 are not livable. 



We have seen that the application of modern ktiozvlcdge 

 about the evolution of organisms has increased our wealth 

 incalculably by establishing varieties of plants and animals 

 that are more resistant to disease or to other unfavorable con- 

 ditions and by establishing varieties that bear more abundantly 

 of those materials for which we care, — for example, more wool 

 in the case of the sheep, more sugar in the case of the sugar 

 beet, and so on. In similar ways the solution of the problems 

 of evolution must continue to contribute to human welfare 

 on the economic side, and probably also on the social and 

 moral side. 



