376 PROGRESSIVE DEVELOPMENT 



Museum of Natural History 

 From fossil evidences, the evolution of the horse has been reconstructed. There 



of years this slow accumulation of variations will lead to new 

 species as well as to a great diversity in one species. 



There are a great many facts to support Darwin's theory of 

 natural selection, although some objections are made to it. (1) 

 Variations are frequently not inheritable, Darwin did not dis- 

 tinguish between somatic and germinal variations. (2) The varia- 

 tions frequently do not have anything to do with the fitness of the 

 organism for its environment. (3) The struggle for existence 

 does not necessarily weed out the unfit organisms. Frequently, 

 the weeding out occurs before the organisms have grown sufficiently 

 to have their characteristic differences appear. Plants and ani- 

 mals are frequently killed off by agencies, forces, and accidents 

 that do not discriminate between the fit and unfit. 



The de Vries theory of evolution. The theory of mutation was 

 formulated by Hugo de Vries, a Dutch botanist, in 1904. He 

 published his ideas in a book called Species and Varieties. Charles 

 Darwin had collected many examples of organisms strikingly 

 different from the other members in the species. These he called 

 sports. He did not attach very much importance to them. Hugo 

 de Vries based his theory of evolution on sports which he called 

 mutants. Weismann's conclusions in regard to germ plasm had 

 disproved the inheritance of acquired characters. He had claimed 

 that modifications of the body or somatoplasm did not affect the 

 germ plasm and therefore could not be inherited. This fitted in 

 with the ideas of Hugo de Vries who had made direct observations 



