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- 

 coccuSj 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- 



