INHERITANCE OF ABNORMAL VENATION. 23 



favored notion that there is some specific substance in the germ which 

 produces the character in question and which is divided at the cell- 

 division which separates the substances forming* the right side from 

 those forming the left side of the complete soma, it would be difficult to 

 believe the division is always, or in most cases, exactly even. If it is not 

 with respect to the abnormality-producing factor, it would give rise to 

 the phenomenon of asymmetry as to the extent of the abnormality. 

 Only when the factor is rather weak to start with would this deviation 

 from exact equality of division frequently result in the share going to 

 one wing being so small that that wing would be normal while the other 

 wing is abnormal; hence there would be a correlation between the 

 degree of the abnormality and the phenomenon of one wing being normal 

 while the other is abnormal (see p. 5) . The approximate equality of 

 the apportionment of the factor in division may be taken as the expla- 

 nation of the correlation in the intensity of the abnormality in the two 

 wings, and the degree of this correlation is a measure of the degree of 

 equality of the division. Since the going of a slightly greater strength 

 to the right side than to the left, or vice versa, is a mere accident in 

 development, it is not to be expected that there will be an inheritance 

 of a particular side getting the greater strength (see p. 17) . But since 

 flies abnormal in only one wing came from germs which had a weak 

 abnormality-producing factor, it is to be expected that the germs they 

 produce will be weak with respect to this factor, and so a smaller per- 

 centage of their offspring will be abnormal than of the offspring of 

 parents abnormal in both wings (see p. 17) . 



Condition 5. — Since it takes a greater strength of the germinal factor 

 to produce abnormalities in the males than in the females, a male 

 somatically normal may be produced by and produce germs containing 

 as strong or stronger abnormality factors than a female which is somat- 

 ically abnormal. Hence, in the long run, normal male X abnormal 

 female will give more abnormal offspring than abnormal male X normal 

 female, because in the latter cross one of the parents (the female) neces- 

 sarily has the factor very weak. 



Condition 6. — "In selecting parents to continue the normal strain, I 

 merely selected flies having no extra veins. For the most of the time 

 the work of describing offspring was unavoidably so far behind the 

 mating work that I did not know what percentage of their brothers and 

 sisters were abnormal. Hence I had no way of judging as to the ger- 

 minal constitution of the parents." Being unable, by examination of 

 the soma, to tell the exact strength of the germinal content, I uncon- 

 sciously used as parents flies in which the abnormality-producing factor 

 was relatively strong, and thus started and for a time maintained a 

 strain giving a relatively large number of abnormal flies. When the 

 flies were allowed to do their own selecting of mates they were more 

 successful (see p. 36) . 



