GENES—STURTEVANT 
plete picture, since it may also be pos- 
sible that both substrate and enzyme 
are present, but some other condition 
(e. g., the hydrogen ion concentra- 
tion) prevents their interaction. 
The relation between different forms 
of the same gene may be interpreted 
along similar lines. The effects of 
these different forms are often not 
parallel in different parts of the same 
individual. There is, for example, a 
gene in the ‘“‘wild type’? Drosophila 
that mutates to produce a type called 
yellow. There are several different 
mutant forms of this gene: yellow—1 
results in yellow bristies and yellow 
wings; yellow-2 causes yellow wings, 
with little effect on the bristles; 
yellow-3 produces yellow bristles and 
dark wings. In such cases as this— 
and they are very frequent—it may 
be supposed that there are related but 
slightly different substrates in differ- 
ent parts of the body, and therefore 
the efficiencies of the slightly different- 
ly shaped enzymes are not necessarily 
parallel. 
If one accepts the view that there 
are fairly numerous slightly different 
genes at many loci, it is probable that 
those which are concerned in particu- 
lar reaction systems need to be ad- 
justed to each other. If one gene is 
working at a high level of efficiency, it 
is probable that the other genes that 
influence related reactions should also 
be working at high levels in order to 
produce a harmonious and properly 
adjusted system, though it might be 
possible to have an equally successful 
organism if all the genes concerned 
were working at a lower level. ‘There 
is some evidence that precisely such 
differences do occur between related 
species. It may be suggested that 
gene systems gradually drift apart in 
their levels of activity during the 
differentiation of species; in that case 
much of the sterility and inviability 
often found in species hybrids and 
their offspring may be due to bringing 
together genes that differ in their levels 
of activity rather than in their specific 
effects on development. 
301 
The view that related forms of 
organisms owe their resemblances to 
the possession of common genes leads 
to an interpretation of homology. 
This is a biological concept that lends 
itself rather easily to somewhat mysti- 
cal speculations. Accordingly it has 
not often been seriously discussed in 
connection with the results of modern 
experimental work. There is, never- 
theless, something real in the relations 
covered by the term, whether one 
considers serial homology within an 
individual or the homologies between 
organs in different groups of organ- 
isms. The point of view here devel- 
oped leads to the interpretation that 
two organs are homologous to the 
extent that their development is con- 
ditioned by the same genes. One 
consequence of such a formulation is 
that homology becomes a relative, 
rather than an all-or-none, phenome- 
non. This result seems to me an 
advantage, since it suggests the possi- 
bility of a quantitative mathematical 
approach. 
If genes do not change their func- 
tions, but only change in the relative 
efficiency with which they carry on 
their predestined ones, it follows that 
organisms also cannot develop new 
functions—which is obviously contrary 
to fact, for there can be no doubt that 
new functions do develop in the course 
of time. It may be taken as probable 
that most of the genes present in an 
organism are performing functions 
that are advantageous to the organ- 
ism, for otherwise they will not long 
persist. ‘This is not a teleological view, 
but one that follows from the observa- 
tion that most changes in genes are in 
the direction of loss of activity. Evi- 
dently the only stable condition of the 
gene composition of a particular locus 
is the absence of the gene in question, 
and unless there is selection for some 
particular function resulting from the 
activity of the genes at that locus, 
there will come to be no gene there. 
Most of the genes, then, are needed 
by the organism, and cannot well be 
spared for the production of new func- 
