22 



EXPERIMENTS WITH DROSOPHILA AMPELOPHILA. 



generation. 



' We have, now, only to take up the fact that in inheritance these 

 abnormalities follow the spirit but not the letter of the Mendelian law 

 (condition 7). We might consider that the dominance of normal over 

 abnormal is merely due to the dilution of the abnormality-producing 

 factor in the NA's. If it is strong it may be potent enough to produce 

 abnormalities in spite of this dilu- 

 tion, thus giving incomplete domi- Tab l le 23.— Results of seven crosses between 

 ■n , ., . abnormal and normal strains in a late 



nance. Even when it is pure 

 (AA), its fluctuation may give 

 individuals in which the zygotic 

 strength is not great enough to 

 produce abnormalities, thus ac- 

 counting for the normals in the 

 abnormal strain. Whether one 

 could so increase the strength of 

 the abnormality-producing factor 

 that when the selected flies are 

 mated with flies lacking the factor 

 all the offspring will be abnormal is not certain, but table 23 indicates 

 such a possibility." 



If, however, we have, in carefully conducted experiments, many flies 

 somatically normal but germinally abnormal, and if by selection it is 

 easy to so weaken the abnormality-producing factor that from a strain 

 100 per cent abnormal we get and keep one 100 per cent somatically 

 normal (all presumably germinally abnormal, since they came from a 

 100 per cent abnormal strain) , must we not admit the possibility that all 

 somatically normal flies have the germinal possibilities of abnormality? 

 This makes the problem much simpler, as, leaving out the question of 

 Mendelian segregation, we have only to consider the inheritance of the 

 variations of an abnormality-producing factor, whatever that may be. 

 Let us take up the seven conditions which must be satisfied. 



Condition 1. — All flies possess the abnormality-producing factor in the 

 germ. It is usually so weak that it has no visible effect upon the soma. 

 Occasionally, however, it is strong enough to do so, and its strength 

 can be so increased by selection that it always does so. 



Condition 2. —It is necessary to suppose that it takes a greater strength 

 of the germinal factor to have a visible effect upon the male soma than 

 upon the female. This sexual difference of developmental physiology 

 is quite common and the hypothesis will doubtless be readily allowed 

 by most critics in this case. It is interesting to wonder whether the 

 possession of horns by certain male ungulates, while the females lack 

 them, is an extreme example of this same phenomenon. 



Condition 3.— To be expected on this hypothesis. 



Condition U. — The explanation here would differ according to different 

 notions of the mechanics of heredity. If we accept the apparently most 



