TYPES OF MENDELIAN HEREDITY Al 
abnormal as though the female had herself been abnor- 
mal. The reciprocal cross, viz., abnormal females by 
normal males, givesabnormal sonsand daughters, if the 
food is suitable, but normal if the food is dry, etc. In 
both cases the F, gives the expectation for a sex-linked 
dominant factor if the medium is suited to bring out 
the abnormal character, and the result is entirely ob- 
scured if the foodisdry. Here, at will, we can demon- 
strate a regular Mendelian ratio by control of the 
environment, and conversely, we can conceal com- 
pletely what is taking place by substituting another 
environment. That thesame genetic process is going 
on in both cases can be demonstrated by suitable 
tests. 
A case similar in principle occurs in a mutant stock 
of Drosophila that produces supernumerary legs. 
This stock was observed in winter to produce a con- 
siderable percentage of flies with supernumerary legs, 
but few or none in summer, especially in warm 
weather. Miss Hoge, who has studied this stock, 
finds that when the flies are kept in an ice chest at a 
temperature about 10° C. a high percentage of flies 
with supernumerary legs occurs. Sometimes several 
legs or parts of a leg are doubled, or the doubling 
may occur twice in the same leg. The general rule 
that Bateson pointed out for duplicated legs in other 
insects appears to hold here, viz., the adjacent parts 
are mirror images of each other. 
In the cold the duplicate leg gives a regular 
Mendelian result; but at normal temperature the 
duplication is a rare.event and its mode of inheritance 
