298 
gene that is consistently present in 
every such chromosome of every 
normal ‘“‘wild type” individual. This 
locus may at times be occupied by a 
distinctly different mutant gene, and 
these constitute the working material 
of the geneticist, but the resulting in- 
dividuals are somewhat aberrant and 
are rather rare. The usual typical 
individual always carries the ‘‘wild 
type’? genes. This is perhaps an over- 
simplified formulation, but it does, I 
think, represent something closely ap- 
proximating the viewpoint of many 
geneticists. 
This view has now been questioned. 
It is clear that, at least for some loci, 
there exist several or many nearly 
equivalent genes, any one of which 
may be present without any marked 
effect on the organism (7). The earlier 
interpretation of a single “wild type” 
gene at each locus was due to lack of 
refinement of the methods of the genet- 
icists. It is not yet clear how wide- 
spread this phenomenon is; it may be 
the rule that at each locus there are 
numerous nearly equivalent genes in 
the ‘‘wild type” individuals, rather 
than only one typical one. 
Some geneticists, impressed by this 
evidence, have been inclined to go a 
step farther and to postuldte rather 
frequent changes from one such gene 
toanother. They feel that perhaps the 
older view of the great stability of genes 
was also only a first approximation to 
the true state of affairs; that perhaps 
the whole system is in a state of flux, 
and in the course of time the individual 
genes may undergo extensive changes 
of function, even to the extent of ex- 
changing their roles in the determina- 
tion of the properties of the organism. 
Perhaps the most extreme statement 
of this view is that of Harland (4) who 
writes: “The genes, as a manifestation 
of which the character develops, must 
be continually changing” and, “‘.. . 
we are able to see how organs such as 
the eye, which are common to all ver- 
tebrate animals, preserve their essen- 
tial similarity in structure or function, 
ANNUAL REPORT SMITHSONIAN INSTITUTION, 1948 
though the genes responsible for the 
organ must have become wholly al- 
tered during the evolutionary process, 
since there is now no reason to suppose 
that homologous organs have anything 
genetically in common.” 
This is a point of view very different 
from that to which I have been led by 
the study of the species of Drosophila. 
It is, however, the opinion of an ex- 
perienced geneticist, and is based on a 
large body of experimental data de- 
rived from species comparisons within 
the genus Gossypium, to which the cot- 
ton plant belongs. That evidence may 
now be examined. 
The wild species of cotton usually 
have a dark purplish or maroon spot 
at the base of each petal. Some of the 
cultivated races lack this spot, but its 
presence is the rule in the wild forms. 
In this group, as in many others, 
species comparisons may be made by 
crossing distinct forms and studying 
the genetic behavior of their fertile 
hybrids. When this is done, it is found 
that the petal spot has a different 
genetic basis in some species. The 
genes responsible for its development 
in one species may be absent in another 
one, but the spot is still present and is 
now determined by different genes. 
Since it is probable that the ancestors 
of these two species, back to their ulti- 
mate common ancestor, all had the 
petal spot, it is evident why the con- 
clusion has been drawn that some 
genes have exchanged functions. The 
argument becomes even stronger when 
it is shown, as has been done, that cer- 
tain other characters of cotton plants 
likewise show differences between the 
species in their genetic determination. 
The facts are, I think, not open to 
doubt, but there is a special reason 
why the conclusion seems question- 
able. 
The New World cultivated cottons, 
from which most of the evidence is 
derived, have 26 pairs of chromosomes, 
which may therefore be conveniently 
represented by the letters of the alpha- 
bet (fig. 2). It is clear from several 
