GENETICS: W. E. CASTLE 
facts as I have described them are now pretty generally accepted as correct, 
but two different views as to their interpretation have been suggested, both of 
which can not be true. These two views were outlined by Castle and Phil- 
lips'^ when the first part of the experimental data was pubhshed. We con- 
sidered the evidence then in hand inconclusive as between the two interpre- 
tations and planned experiments to yield, if possible, decisive evidence for one 
or the other. This evidence is now complete, but before I undertake to sum- 
marize it, I wish to outline the alternative interpretations to be tested. They 
center about the concept of the 'gene,' to which reference has already been 
made. The term gene or gen was introduced by Johannsen in an attempt to 
simplify the ideas involved in the previously current term, unit-character. 
By unit-character was understood (1) any visible character of an organism 
which behaves as an indivisible unit in Mendelian inheritance and (2) by im- 
plication, that thing in the germ-cell which produces the visible character, 
Johannsen^ pointed out that these two things were logically distinct, suggested 
the term gene for the hypothetical germ-cell determiner, and made it clear that 
it is not possible to say how many germinal determiners (genes) are involved 
in the production of a single visible character, but only hovv^ many are present 
in alternative forms (as allelomorphs). He therefore advised the entire dis- 
continuance of the use of the term unit-character and proposed to discuss the 
subject of heredity exclusively in terms of genes. This is the so-called geno- 
type theory. 
Before this theory could be accepted unreservedly, it has seemed desirable 
to know whether all observed inheritance phenomena can be expressed satis- 
factorily in terms of genes, which are supposed to be to heredity what atoms 
are to chemistry, the ultimate, indivisible units, which constitute gametes 
much as atoms in combination constitute compounds. It also seemed de- 
sirable to know whether a single gene is indeed invariable like an atom (or a 
simple chemical compound). 
Much study has in recent years been given to these questions with the re- 
sult that (1) to express all heredity in terms of unvarying genes, it is necessary 
to suppose that besides the single gene indispensable to the production of a 
visible character, its gene proper, there occur also other genes whose action is 
subsidiary. Their action may not be indispensable to the production of a 
character, yet they certainly modify its visible form. These are called modi- 
fying genes. In some cases they are known to have other functions also. 
Thus the gene proper of one character may function also as a modifying gene 
for another character. But in the majority of cases the only ground for hy- 
pothecating the existence of modifying genes is the fact that characters are 
visibly modified. 
As an alternative to the theory of modifying genes, the theory has been con- 
sidered that genes may themselves be variable and if so, genes purely modi- 
fying in function might be dispensed with. 
