34 GENETIC VARIATIONS 



ploids). All these different types live and develop; they are 

 known in many organisms. Figure 7 represents also other 

 types that live: those in which one or two pairs have an addi- 

 tional chromosome, and the like. 



By adding or subtracting one or two chromosomes to the 

 different groups ("pairs") in these tetraploids or other types, 

 and in other ways, a great number of varieties are producible 

 from a single type. Blakeslee and his associates have produced 

 and studied in this way eighty-nine different varieties in 

 Datura, the Jimson weed. And these are but a fraction of 

 those that are possible. Computations show that in Datura 

 there must be three thousand six hundred and twenty differ- 

 ent possible types producible from the single typical variety, 

 by thus altering the grouping of the chromosomes and genes. 

 There is reason to believe that all of these can be produced 

 and that they will live. 



The conditions thus carefully studied in Datura are not 

 exceptional; they are found in many other organisms, par- 

 ticularly in plants. They have been very extensively studied 

 also in Oenothera, the evening primrose. Here again there are 

 many different varieties or types, resulting from changes in 

 the grouping of the chromosomes and genes. You will re- 

 member that it was on the evening primrose that Hugo De 

 Vries made the famous studies that he published about the 

 year 1900 in the great two-volume work, Die Mutationsthe- 

 orie: a work which may be said to constitute the beginning 

 of modern genetics. Most of the changes in inherited charac- 

 teristics that De Vries described — what he called mutations — 

 are now known to be the result of changes in the grouping of 

 the chromosomes and genes, of the sort that we have sketched. 



Most of the varieties produced thus by altering the numbers 

 and grouping of the chromosomes are not entirely permanent 

 when reproduction occurs from two parents. The irregulari- 



