1922] CURRENT LITERATURE 227 
the “balanced” tetraploid (or indirectly, triploid) types. The tetraploid types 
breed true, but the others produce (in various proportions) several types of 
progeny, including individuals like themselves and others like the “normal” 
(pure diploid) original ancestors 
MULLER takes up 5 Sy in the individual gene (“‘locus ie or true 
mutations), and discusses their general characteristics. It is rtant to 
realize that the change is not always a mere loss, for clear-cut reverse Sl nea 
have been obtained in corn, Drosophila, and Portulaca, If the original muta- 
tion was a loss, the reverse mutation must be a gain. “It is generally true 
that mutations are much more apt to cause an apparent loss in character than 
a gain, but the obvious explanation for that is, not because the gene tends to 
lose something, but because most characters require for proper development a 
nicely adjusted train of processes, and so any change in the genes, no matter 
whether loss or gain, substitution or arrangement, is more likely to throw 
the swyelopineatal mechanism out of gear, and give a ‘weaker’ result, than to 
intensify it.” MuvLLER depicts a very interesting and suggestive analogy 
between the gene and certain immunity reactions. 
RIDGES’s elucidates the following very significant thesis. The characters 
of an organism, instead of being absolutely “determined” by a single gene, 
should rather be thought of as being acted upon simultaneously by many genes. 
particular character will be determined by the equilibrium between its modify- 
ing genes. The justification for this thesis appears from a consideration of 
some of BrincEs’ non-disjunctional Drosophilas, which exhibit previously 
unknown grades in the expression of a number of characters. Most startling 
are the cases where the character involved is sex itself; so that the fruitfly, pre- 
viously the best known example of qualitative differentiation of sex on the basis 
of the X and Y chromosomes, now provides the most promiaing example of a 
pearls sex mechanism with the newly discovered “intersexes” and 
“supe 
EMERSON” presents os classifies a great mass of slag on a variation. 
He considers separately “somatic mutation of genes” and “ ic segrega- 
tion,” and under the era heading ‘“‘ chromosome daalagtion” . toler 
segregation,” and “graft hybrids and other chimeras.” This article should be 
unusual interest to arenas 
* Mutter, H. J., Variations due to change in the individual gene. Amer. Nat. 
5632-50. 1922 
*s BripcEs, CALVIN B., The origin of variations in sexual and sex-limited charac- 
ters, Amer. Nat. 56:51-63. figs. 7. 1922. 
6 Bor. Gaz. 72:408-410. 1921. 
77 Emerson, R. A., The nature of bud variations as indicated by their mode of 
inheritance. Keene, Nat. 56:64-79. 1922. 
