12 THE METHODS AND 



tains. The stem of the other, one of de 

 Tries' famous mutations, snaps at once like 

 short pastry, because it does not contain the 

 factor for the formation of the fibres. Such 

 plants may be sister-plants produced by the 

 self-fertilisation of one parent, but they are 

 distinct in their composition and properties 

 and this distinction turns on the presence 

 or absence of elements which are treated as 

 definite entities when the germ-cells are 

 formed. When we speak of such qualities as 

 the formation of pigment in an eye, or the 

 development of fibres in a stem, as due to 

 transmitted elements or factors, you will per- 

 haps ask if we have formed any notion as to 

 the actual nature of those factors. For my 

 own part as regards that ulterior question I 

 confess to a disposition to hold my fancy on 

 a tight rein. It cannot be very long before 

 we shall know what some of the factors are, 



