156 LUTHER BURBANK 



In studying races of animals and plants, biol- 

 ogists have discovered that this tendency, spoken 

 of as tendency to revert to a mean, is universal. 



The matter has been especially studied in 

 recent years by the Danish biologist, Professor 

 W. L. Johannsen, of Copenhagen. His studies 

 of barley and of kidney beans show that any 

 given race of these plants is really made up of a 

 number of subordinate races, representing differ- 

 ent strains of the ancestral pedigree, and that 

 when the plants are self-fertilized, the progeny 

 tend to group themselves into a few more or less 

 permanent types. 



There are limits of variation as to size, color 

 and qualities, but the progeny as a whole do not 

 tend to have offspring that approach the halfway 

 mark between these two extremes. Rather they 

 break up into groups, each group tending to 

 reproduce itself in such a way as to form a new 

 subordinate race or "pure type." Thus from the 

 same mixed stock sundry races of relative giants 

 and of relative dwarfs, as well as numerous inter- 

 mediate races, are formed. 



Now it would appear that such a case as that 

 of the prune, in which we are able to work out 

 by artificial selection a race characterized by 

 tendency to early fruitage, is in keeping with 

 these studies of the so-called "pure lines" of de- 



