HEREDITY 543 
expression of the inheritance for the given environment. 
Under different external conditions the expression might 
be different; but the inheritance -would be the same. The 
chlorophyll in the daughter-cells, immediately after cell- 
division, is a direct inheritance, but the chlorophyll subse- 
quently manufactured, and the green color which it gives 
to the plant, are not inherited; they are expressions of the 
inheritance which in this instance is a chloroplastid that 
reproduces itself by division, and manufactures chlorophyll 
in the presence of sunlight. Under abnormal external 
conditions the mechanism may not act, or may act ab- 
normally, so that yellow pigment appears instead of green 
or in darkness no pigment at all. In either case the in- 
heritance is the same, but the expression varies. A mod- 
ern writer has denned inheritance as all that an organism 
has to start with. It is the protoplasmic substance, with 
all its potentialities, passed on from parent to offspring. 
464. Inheritance Versus Expression. In the light of 
this information, obtained by a study of the lowly Pleuro- 
coccus, we are able to understand that what we inherit 
from our parents or grandparents, is not a certain shape of 
nose, a certain characteristic gait, a musical or mathe- 
matical bent of mind, a quick temper, but a substance 
(protoplasm) possessing a very delicate, intricate, and 
characteristic constitution or mechanism. Under certain 
conditions this inheritance may so express itself as to 
cause resemblance in some physical or mental trait; or it 
may find a quite different expression, as when parents of 
medium height have tall children, or parents musically 
inclined have children that do not care for music; or sweet- 
peas having white flowers only, produce, when crossed, 
peas having colored flowers. Or again, not all that is in- 
