226 PHYSIOLOGICAL GENETICS 



enl members of the vestigial scries in Drosophila (except the 



lowest and highest) and for the color of the intersexual males of 

 Lymantria. In these eases, the interpretation is obvious: the 

 asymmetry is a product of the variability in detail of local 

 systemic conditions which regulate the flow of a growth sub- 

 stance (or other such stuff) spreading from the basis of the wing 

 (see page 222). In other cases, the same or a similar process 

 might furnish the explanation, and the authors who have studied 

 such eases have expressed themselves in a more general way 

 which might easily be converted into a picture of the same type 

 that we developed above. Thus, Haecker (1925) spoke of 

 variations in the internal conditions of the system. Wright 

 (1918), studying the asymmetries in color patterns of guinea 

 pigs, spoke of local factors controlling irregularities in develop- 

 ment. The same author (1935) mentions local conditions as 

 causing the asymmetrical expression of Polydactyly in guinea 

 pigs. 



A different category within this group of phenomena may be 

 assigned to the hereditary asymmetry studied by Breitenbecher 

 (1925) in Bruchus (bean weevil). Here a mutant was found that 

 is inherited as a sex-controlled recessive (limited to females but 

 carried by both sexes) in which one elytrum carries two black 

 spots; the other, two red ones. Dextral and sinistral individuals 

 occur in equal numbers, so that it must be assumed that a 

 bifurcated chance process is decisive. It is very difficult to 

 formulate a proper explanation for this type of gene-controlled 

 asymmetry, except by going back to the first stages of develop- 

 ment of wings. (A parallel case in Drosophila is being investi- 

 gated by the author.) 



The oldest example of this type w r as already known to Darwin 

 and has been studied genetically by Przibram (1908). In cats, 

 individuals occur with one blue and one yellow eye. This 

 asymmetry is hereditary, and among the offspring of individuals 

 with blue on the right side individuals with blue on the left may 

 also occur. The inheritance of extra toes in poultry and guinea 

 pigs is of the same type. Many cases of this type occur in man 

 where much work has been done, as the problem becomes impor- 

 tant when identical twins are involved. Danforth, Newman, 

 Dahlberg, and others have discussed these problems and pre- 

 sented facts. Dahlberg (1929) has tried to formulate an explana- 



