22 EXPERIMENTS WITH DROSOPHILA AMPELOPHILA. 
‘“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- TABLE 23.—Results of seven crosses between 
2 eae _ abnormal and normal strains in a late 
nance. Even when it is pure generation. 
(AA), its fluctuation may give | 
Mating | Ab- | Percent | 
individuals in which the zygotic No. normal, | 7% | abnormal. | 
strength is not great enough to | 9561... ag?| ae? ag Pe 
produce abnormalities, thus ac- ae Pence = | rh Le | 
counting for the normals in the 2630... 47 | 79 59.5 
abnormal strain. Whether one 2) seneeeeee | of | zs Cyt 
could so increase the strength of Spine cid 14 24 58.3 
the abnormality-producing factor | SS eee 
that when the selected flies are [pte ze Pat dosage 
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 
ean 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 4.—The explanation here would differ according to different 
notions of the mechanics of heredity. If we accept the apparently most 
