ORGxWIC EVOLUTION 35 



due to one change or to a thousand he cannot with 

 any certainty tell anythino^ ahout the hereditary 

 units which haye made the process of eyolution j)os- 

 sible. Without this knowledge there can be no satis- 

 factory understanding of the origin of new heredi- 

 tary yariations through which eyolution is possible. 



The Evidence from Genetics 



In recent years a study of the orioin of new 

 characters has led to the discoyery that sudden 



a. 



changes appear at times in the germinal elements — 

 changes that haye an effect on one or more charac- 

 ters of the organism. The ^^rocess by which the 

 change takes place is called mutation, and the new 

 kind of indiyidual it produces is called a mutant. 

 The mutants breed true to the new type. How far, 

 and in what sense, the occurrence of mutants suf- 

 fices to supply the material for eyolution in animals 

 and plants is one of the important biological prob- 

 lems of today. 



While in a strict sense genetics deals only with 

 the mode of transmission of the hereditary elements 

 (the genes) from parent to offspring, the scientific 

 study of the origin of new yariations is undertaken 

 today almost exclusiyely by students of o^enetics, 

 and is now recognized as a part of their Avork. I shall 

 include, therefore, under the "eyidence from genet- 

 ics" the origin of new characters by mutation. 



In many groups of animals and plants new kinds 



