226 GENETICS AND EUGENICS 



lomorphic states of the "bar" gene is as follows: Full 

 (normal) eye, 849^.8 facets; bar eye, 75.6 facets; ultra-bar 

 eye, 23 facets. Zeleny also observed lesser mutations of the 

 gene for bar, which made their appearance during 42 care- 

 fully controlled generations of selection for low and for high 

 facet number in brother-sister matings. Aside from muta- 

 tions in the gene for bar, Zeleny observed genetic differences 

 between his high selected and low selected lines which he 

 ascribes to "accessory factors outside of the sex chromosome 

 in which the bar gene is located." These when present in 

 heterozygous state are speedily sorted out by selection, 

 which ** ceases to be effective after three to five generations.'* 

 ** There is, however, no limit to the possibilities of selection 

 if the occasional mutants are included in the series, and two 

 at least of these, reversal to full and ultra-bar, have been 

 shown to be changes in the bar gene itseK." 



We may conclude that the amount of genetic change 

 which is occurring at the present time is greater as regards 

 some characters than as regards others, and is probably 

 greater in some organisms than in others. But in any group 

 of organisms capable of interbreeding, which has been divided 

 for any length of time into non-interbreeding groups (races, 

 breeds, or strains) genetic differences of one sort or another 

 will probably be found to have arisen, when an intensive 

 study of the matter is made. If so, we must conclude that 

 genetic changes are probably occurring with appreciable 

 frequency in most, if not in all organisms. But it should be 

 stated emphatically that the amount of variability to be 

 detected by selection within pure lines is in all cases small 

 as compared with that which can be secured by crossing 

 different strains, breeds or varieties, which have long been 

 established within the species. For in pure line selection only 

 genetic changes occurring during the process of selection are 

 likely to be revealed, but following a variety cross, all pos- 

 sible recombinations may be expected of the genetic changes 

 which have occurred since the two parent groups diverged 

 from each other. 



