No. 609] 



THE THEORY OF THE G 



517 



cleavage process of the egg, is no loiiuci ncccplablc ; for 

 there is direct evidence to show that the wlioh- hereditary 

 complex goes to every cell in the body. This conclusion 

 has the most far-reaching consequences for our present 

 views as to how factors produce their effects in the de- 

 veloping organism, for it follows that the machinery 

 that separates the inherited material into its component 

 elements is not the same mechano-chemical process that 

 brings about differentiation in the embryo. 



So far I have s])oken of the genetic factor as a unit in 

 the germ plasm whose jnesence there is inferred from the 

 character itself. AVhy, it may be asked, is it not simpler 

 to deal with the cliaracters themselves, as in fact Mendel 

 did, rather than introduce an imaginary entity, the gene. 



There are several reasons why we need the conception 

 of gene. Let me illustrate by examples:— 



1. The MaulfoJd Effects of Each Gene 

 If we take almost any iimtniii race, sudi a> \vhite_^es 

 in Drosnphihi, we find tliat the whit(^ eye is only one of 

 tlie cliaracteristics that su.-li a mutant I'acc >iiows. In 

 the piVM-nt ca^.' the solul.ilitv of tlu- yellow pigment of 

 the body i> al>.. alVeeted: the productivity of tlie indi- 

 vidual also: and the viability is lower thaii in the wild 

 fly. All of tliese pe.-nliarities are r.)und whenever the 



from the white eye condition. It follows that whatever 

 it is in the germ pla>m that ].roduce< white eyes, it aNo 



in the germ plasm may ])roduce many kinds of effects on 



