322 GENETICS 



types or varieties producible from the typical diploid, by 

 altering the grouping of the chromosomes in the ways set 

 forth above. There is reason to believe that all of these can 

 be produced and that they will live. 



The conditions in Datura are not exceptional. They are 

 found in many other organisms, particularly in plants. They 

 have been extensively studied in various species of Oeno- 

 thera, the evening primrose. In this plant, as in Datura, 

 there are many different types or varieties, resulting from 

 changes in the grouping of the chromosomes and genes. 



It was in one of the species of the evening primrose, 

 namely, Oenothera lamarckiana, that Hugo De Vries made 

 his famous investigations which were published in about 

 the year 1900 in his two volume work known as Die Muta- 

 tionstheorie, a work which may be said to constitute the be- 

 ginning of modern genetics. Most, though not all, of the 

 changes in inherited characteristics which De Vries de- 

 scribed, and which he called mutations, were the result of 

 changes in the grouping and numbers of the chromosomes 

 and genes, of the sort described above, although this fact 

 was not discovered till much later. 



Non-disjunction and the phenomena resulting from it 

 occur in animals as well as in plants. In Chapter IV an ac- 

 count was given of some of the phenomena of this char- 

 acter in Drosophila. On the whole, production of diverse 

 varieties in this manner seems to be less common in animals 

 than in plants. 



Most of the varieties thus produced by altering the num- 

 ber and grouping of the chromosomes are not permanent 

 in biparental reproduction; that is, the peculiar character- 

 istics of the parents are not fully reproduced in the descend- 

 ants. This is because the irregularities in the chromosome 

 numbers cause irregularities in germ cell formation and 

 fertilization, with the result that the offspring have differ- 

 ent chromosome groupings from those found in the parents, 



