88 Heredity. 



body consists in part of structures which are in no sense 

 alive, but which are built up by the formative activity of 

 the living protoplasm. The shell of a snail or of an oyster 

 is purely inorganic, and although it is built up by the 

 animal, and is necessary to its existence, it is no more a 

 part of the living substance of the animal than the shell 

 which is picked up and inhabited by a hermit crab. It 

 is true that the oyster's shell is formed by the animal, 

 as part of itself, but the shell does not grow, like living 

 tissues, by the absorption and transformation of nutri- 

 ment, but by the crystallization of the amorphous min- 

 eral matter which is poured out by the living cells of 

 the mantle; and microscopic examination shows that it 

 is not an organized tissue made up of cells, but an ag- 

 gregate of purely mineral crystals. 



Since this is the case it is clear that it is not the shell 

 itself, but a tendency to build the shell, which is hered- 

 itary, and is contained in the Qg%\ and an illustration 

 will serve to show that the inheritance of the tendcFicy 

 involves much less complexity in the structure of the ^gg 

 than the inheritance of the thing itself would imply. 



A bee inherits a tendency to build up a comb of wax, 

 and to fill the cells of this comb with honey. 



The comb and the honey are due to the vital activity 

 of the bee. Just as the shell is the result of the vital 

 activity of the oyster; but the statement that the bee's 

 ^gg contains something which corresponds to the struc- 

 tural organization to which the tendency is due, is cer- 

 trinly not equivalent to a statement that the actual 

 comb, filled with honey, is represented in the ^gg. 

 This is just as true of structures which are built up, 

 inside the body, by its vital activity, as it is of those 

 which are built up in the same way outside the body. 



When we take into account all structures of this kind 



