THE TREND OF EVOLUTION 155 



might be of evolutionary value. We now know that fluctua- 

 tions have no evolutionary value; Mendelian combinations 

 probably play a secondary part in supplying the materials of 

 evolution, though this part is not negligible ; mutations, on the 

 other hand, are the fundamental and initial steps in evolution. 

 Fluctuations and new Mendelian combinations occur in count- 

 less numbers ; indeed, they may be said to be universally present 

 among organisms; mutations, on the contrary, are relatively 

 rare. Nevertheless, they are by no means uncommon; during 

 the past dozen years Professor T. H. Morgan and his asso- 

 ciates have found and studied about four hundred mutations 

 in the pomace fly, Drosophila; few of these were beneficial 

 changes and almost all of them would have disappeared in a 

 state of nature, but this probably indicates that natural species 

 and varieties are the products of very severe selection, that 

 thousands of mutations have been eliminated where one or a 

 few survive. Many of our numerous breeds of domestic ani- 

 mals and cultivated plants have appeared as mutations, and 

 while we cannot assume that all that have been preserved are 

 intrinsically useful nor that all useful mutations have been pre- 

 served, the great number of breeds that have arisen among 

 domestic animals and cultivated plants indicates that useful 

 mutations occur in sufficient numbers to furnish the materials 

 for evolution. 



There can be no question that the same fundamental prin- 

 ciples are involved in the evolution of man as in the evolution 

 of any other organism. Inheritance and variation, fluctua- 

 tions, Mendelian combinations, and mutations occur in the 

 human race, just as in plants and animals. Furthermore, there 

 is good evidence that this is true not merely of the body but 

 also of the mind and society of man. The fundamental prin- 

 ciples in all kinds of evolution are similar and if mutations and 

 new Mendelian combinations, but not fluctuations, are the ma- 

 terials of physical evolution, and natural selection is the 



