INTRODUCTION 11 



character may be produced there must be a modification of the 

 germ plasm. The real question, then, is what causes germinal 

 change? In considering this question we must keep in mind the 

 possibility that agencies which are of little importance from the 

 standpoint of the plant breeder may be of profound importance 

 in evolution. 



DeVries' Mutation Theory. The more recent theory of evo- 

 lution developed by DeVries attacks the question of the sort 

 of variations which furnish the basis for evolution. DeVries 

 gives only slight value to the small continouus variations and 

 advances the hypothesis that large variations are of primary 

 value. He believes in periods of mutation when from some un- 

 known cause a species is producing many new forms, and other 

 periods when stability of the species is the rule. DeVries recog- 

 nized three sorts of mutations; (1) progressive, when an entirely 

 new character appears ; (2) degressive, the appearance of a par- 

 tially latent or hidden character; and (3) retrogressive, when an 

 active character becomes latent. The cause, or causes, of these 

 sudden changes was not known. Mutations are frequently not 

 large but small. All sudden heritable changes which cannot be 

 explained by the laws of segregation and recombination are 

 called mutations. 



The Pure -line Theory. The studies of Johannsen are of par- 

 ticular value from the standpoint of the plant breeder. He 

 worked with self -fertilized crops and found that while the progeny 

 of a single self-fertilized plant varied quite widely, these varia- 

 tions were not inherited. From single commercial varieties he 

 found it possible to isolate numerous lines which in their means 

 differed slightly from each other and which bred true. Johannsen 

 considered a pure line to be the progeny of one or more self- 

 fertilizations from a single homozygous ancestor. Selection 

 within such a pure line was of no practical value. Numerous 

 investigations with self -fertilized crops have been made and corrob- 

 orate the results of Johannsen. Isolated cases of mutations in 

 these pure lines have been reported, and while these are of much 

 scientific interest they occur far too infrequently to be used as a 

 basis for a system of breeding. 



Johannsen '& pure-line theory has been extended to cover 

 clonal or asexual propagation in both plants and animals. At its 

 proper place evidence will be given to show that in heterozygous 

 organisms which are asexually propagated there sometimes occur 



