The Evolution and Function of Genes’ 
By A. H, Sturtevant, California Institute of Technology 
Every individual organism is the 
resultant of the activity of the ultimate 
hereditary units, or genes, that it 
carries and of the environmental con- 
ditions under which it has developed. 
The geneticist is likely to lay particular 
emphasis on the gene as the hereditary 
component in this dual control, but 
it must never be forgotten that the en- 
vironment is also essential. The dif- 
ferences between individuals may be 
due to differences in either genes or 
environment. There is one group of 
characters where caution in interpre- 
tation is especially necessary, namely, 
those having to do with human 
faculties. 
There can, I think, be no doubt that 
there are differences between people 
in their inherent, inherited, mental, 
and psychological potentialities; there 
can also be little doubt, by analogy 
with other characters, that there are 
at least statistical differences between 
races. But precisely these characters 
are obviously peculiarly sensitive to 
environmental effects—to tradition 
and to social and economic conditions. 
It seems clear, therefore, that one can- 
not conclude that there is a very high 
correlation between the _ inherited 
mental potentialities of an individual 
and the properties that he actually 
possesses, and this caution is doubly 
necessary when one compares different 
races. 
1 From American Scientist, vol. 36, No. 2, 
1948 (reprint, Science in Progress, Series 
VI, 1949), and here reprinted by permission 
of The Society of the Sigma Xi and the 
Yale University Press. 
I 
An individual belonging to any of 
the higher plant or animal groups car- 
ries numerous different genes. The 
exact number cannot be specified, for, 
while there are several methods of ar- 
riving at estimates of the number, none 
of the methods is very satisfactory. It 
seems safe to suppose that the number 
is at least in the thousands, but it may 
well be in the tens of thousands. Per- 
haps the next question that a physicist 
or achemist might ask about the genes, 
after their number, would be about 
their size. It is clear that they are 
small, but again an exact answer Is 
not possible. They are evidently so 
small as to be beyond the resolving 
power of an ordinary miscroscope; 
they are probably within the size 
range of large complex organic mole- 
cules, but it is not possible to be more 
specific. 
In spite of these uncertainties with 
respect to such basic matters as their 
number and their size, genes are im- 
portant units, for they are responsible 
for all the characters present in living 
things, and it is, therefore, desirable 
to find out all we can about them. 
One of the many methods that has 
been used in the study of genes is the 
comparative one. As a result we now 
have reasonably satisfactory accounts 
of the genetic behavior in a wide vari- 
ety of organisms. Several kinds of 
vertebrates and of insects are well 
understood, there is some information 
on a few other invertebrate animals, 
extensive data are available on many 
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