282 Heredity and Environment 



thera (Fig. 99) seem to be of this sort as Gates, Miss Lutz and 

 Stomps especially have emphasized; for example O. lamarckiana 

 has 14 chromosomes, O. lata, O. albida and O. scintillans 15 each, 

 O. semi-gigas 21, O. gigas 28, and these variations in the number 

 of 'chromosomes are probably due to abnormalities in their divi- 

 sions, such as non-disjunction, irregular distribution, or chromo- 

 some division without cell division. It is significant that the 

 mutants lata and semi-gigas have occurred several times, whereas 

 gigas appeared but once; this may be explained by the fact that 

 the chances of the doubling of chromosomes in both germ cells 

 (gigas) are very few compared with the chances of their doub- 

 ling in one germ cell (semi-gigas) or of their increase by one in 

 a single germ cell (lata). 



But it is probable that mutations are not usually associated with 

 changes in the number of chromosomes. Where the number of 

 chromosomes remains constant the change may take place in the 

 composition of single chromosomes, as in "cross-overs" or in the 

 number or composition of the chromomeres or units of the next 

 lower order. But such changes as these concern only the emer- 

 gence into visibility of more fundamental changes, for whatever 

 the cellular changes may be which accompany mutations, it is 

 certain that the fundamental changes take place in the inheritance 

 factors themselves. 



In addition to gene mutations, such as the transformation of 

 the gene for the red eye of Drosophila into one for white eye, 

 some genes appear to have become inactive or to have dropped 

 out altogether, as in the white sweet peas shown in Fig. 33, in 

 the wingless or eyeless mutants of Drosophila (Figs. 101-103), 

 and in many other cultivated races of plants and animals, thus 

 producing regressive mutations. Indeed most of our domestic 

 animals and cultivated plants appear as if they had arisen by the 

 omission of something which their wild ancestors had. It was 

 this appearance of omission which led to the "presence and ab- 

 sence hypothesis" (p. 97) which has now been abondoned. Phe- 



