UNFIXABLE CHARACTERS 113 



non-yellow zygotes as 2:1, the ratio observed also among the 

 surviving young of yellow by yellow parents. 



This interpretation of the 2:1 ratio observed in this case is 

 strongly supported by a similar case among plants, in which 

 the evidence is even more complete. A so-called " golden " 

 variety of snapdragon, one in which the foliage was yellow 

 variegated with green, was found by the German botanist, 

 Baur, to be unfixable, producing when self-pollinated fully 

 green plants as well as golden ones, in the ratio 2 golden: 

 1 green. The green plants were found to breed true, that 

 is, to be recessives, while the golden ones were invariably 

 found to be heterozygous. Baur found, however, by ger- 

 minating seeds of golden plants very carefully, that there 

 were produced in addition to green plants and golden ones a 

 few feeble seedlings entirely yellow, not variegated with 

 green, as the golden plants are. These, for lack of assimilat- 

 ing organs (green chlorophyl), straightway perished. Clearly 

 they were the missing pure yellow zygotes. 



Frequently one of the visible characters of an organism 

 depends upon the combined action of two or more inde- 

 pendent Mendelian factors, in which case it is demonstrably 

 not a unit-chaTacteT, as has already been pointed out, since 

 each of the known " factors " is indispensable to the develop- 

 ment of the visible character, as are probably also a great 

 many other as yet unknown factors. The dependence of a 

 visible character upon two or more simultaneously varying 

 factors leads to the production of modified dihybrid or poly- 

 hybrid F2 ratios. It also leads to a phenomenon known as 

 atavism or reversion y by which is meant the restoration of a 

 lost ancestral character, which frequently follows crossing of 

 unrelated varieties. 



Atavism or reversion to an ancestral condition is a phe- 

 nomenon to which Darwin repeatedly called attention. He 

 realized that it is a phenomenon for which general theories 

 of heredity must account. He supposed that the environ- 

 ment was chiefly responsible for the reappearance in a species 

 of a lost ancestral condition, but that in certain cases the 



