354 The Unity of the Organism 



a, 



. in the genus Oenothera the original number of chro- 

 mosomes is 14. This is true of Oe. Lamar ckiana and many 

 other species. Duphcation of one of these chromosomes 

 through an irregular mitotic division has led to 15 in Oe. 

 lata, a characteristic mutation which has occurred both in 

 Oe. LamarcKiana and in certain races of Oe. hiennis. The 

 same chromosome number occurs in semilata and in a very 

 different form from Sweden which I have called incurvata. 

 De Vries has recently described still another form having 15 

 chromosomes. It was derived from Oe. biennis semi-gigas 

 pollinated in part from Oe. biennis . . . Hence wc may say 

 that whenever a germ-cell having 8 chromosomes fertilizes 

 a normal germ-cell a new form is produced. . . . Oe. mut. 

 gigas is a prototype of another series of still more closely 

 parallel mutations in which the chromosome series is doubled 

 — 28 — the plant being a cell giant and not merely gigantic 

 in its external dimensions. ... A third series of morpholog- 

 ical mutants is the semigigas series, having 21 chromo- 

 somes. . . . Another important feature of mutations which 

 has not hitherto been emphasized is the fact that each is the 

 result of a cell change which is represented in every part of 

 the organism. The cells of Oe. lata constantly have 15 

 chromosomes, in whatever part of the plant they have been 

 examined. Similarly in Oe. gigas even the most specialized 

 tissues retain the double number of chromosomes transmitted 

 to them." 21 



The examples of connection between mutations and chro- 

 mosomes which are now attracting most attention are fur- 

 nished by the fruit flies (DrosophUa), Biology is especially 

 indebted to Professor Morgan's genius for experimentation 

 for the investigations in this field. The explanation of the 

 behavior in heredity of mutant attributes in DrosophUa 

 elaborated by Morgan and his co-workers is admittedly 

 hypothetical and is consequently not really entitled to a 

 place in this section, the aim of which is to present cases of 



