The notion that human tumor development depended on a
succession of gene mutations was enormously satisfying be-
cause it echoed a theme that had been reverberating in the
halls of science for more than a century. Tumor develop-
ment showed striking parallels to the evolution of species.
In the mid-nineteenth century, Charles Darwin had de-
scribed evolution in terms of nature's ability to select the
fittest from among a heterogeneous population of organisms.
After the discovery of gene mutations in the 1920s and
1930s, Darwin's theory of natural selection was refined and
extended. Now scientists realized that randomly occurring
mutations created genetically heterogeneous populations of
organisms, and that natural selection chose among these, fa-
voring the survival and reproduction of those organisms
that happened to carry the most favorable constellations of
genes.

An analogous process seemed to be playing itself out
within human tissues. In this instance, the competing life
forms were individual cells. A cell that happened to sustain
a mutation altering one of its growth-regulating genes might
have a growth advantage over its genetically normal neigh-

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