126 W. R. B. ROBERTSON 
and animals, melanism in rodents, retarded mental development, 
feeblemindedness in man, dwarfisms, etc., abnormal deviations 
from the type of the species which may be classed as deficient 
characters? 
The amount which may be dropped from one chromosome, 
such as I have shown in Tettigidea, may be considered too much 
to allow of the existence of the organism in the homozygous con- 
dition, 1.e., where both members of the pair lack it. It is con- 
ceivable that a limit in the amount that may be absent might 
occur and that, going beyond that limit, the homozygous con- 
dition might be lethal, as in yellow mice. Here the homozygote 
always dies, or may never be formed, as has been shown by the 
results obtained from breeding yellows together. The litters 
are three-quarters the normal size and the young are found to 
be two-thirds yellow and one-third gray or other colors, never 
breeding true to yellow. This indicates that the yellow parents 
were heterozygotes and that the one-fourth pure yellows, which 
we would expect in F., have never been formed. Baur (07) 
found, on crossing two varieties of snapdragon—the green leaved 
with the golden leaved—that the offspring were 50 per cent green 
and 50 per cent golden leaved. The greens bred true but the 
golden variety produced 25 per cent greens, 50 per cent golden 
and 25 per cent that were almost white. The latter died at the 
end of germination, when the food in the seed was used up, since 
they possessed no chlorophyll. The golden variety was varie- 
gated with green patches (chlorophyll-containing cells) and so 
could manufacture its own starchy food. 
The yellow condition in mice and in the snapdragon may be 
due to a greatly deficient chromosome, so greatly deficient that 
a zygote having two such deficient chromosomes might have too 
great a deficiency to be able to develop, or, after having developed, 
may lack some substance, such as the chlorophyll in the snap- 
dragon, necessary for carrying on one of the vital life processes. 
In addition, this lack might be so great that it would over-ride 
the normal chromosome in the heterozygous zygote, giving a 
yellow mouse instead of a gray, black, or brown, as the case may 
be; or in the snapdragon a golden variety instead of a green 
