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Aristotle (384–322 BCE), another Greek philosopher,
challenged Hippocrates’s theories of inheritance. He noted that a
man who is missing a left arm could still father children with two
arms. How could that happen if the father had no left arm traits
to pass down? Aristotle also realized that people can pass down
such characteristics as graying hair or male pattern baldness
that don’t show up until their children become adults. Aristotle
concluded that children must inherit some kind of biological
information from both of their parents.

NATURAL SELECTION
Over the next two thousand years, scientists and philosophers
had many ideas about heredity. Some believed that a human
organism started as a preformed individual, in either the
sperm or the egg, and it developed and enlarged in the womb.
Others accepted the theory of epigenesis, the idea that an
embryo is not preformed but develops in stages. It wasn’t until
the nineteenth century that two scientists, Charles Darwin
and Gregor Mendel, gave the world more scientifically based
theories of heredity.
English geologist and biologist Charles Darwin (1809–1882)
worked on a ship, the HMS Beagle, which sailed to chart the
waters of the South American coast in 1831. During his almost
five-year voyage, Darwin collected specimens and wrote down
his observations of their natural processes, which he sent back
to the scientific community at Cambridge University in England,
where he had studied. From his observations, Darwin recognized
an important mechanism of inheritance that he called natural
selection. He observed that those organisms whose physical
characteristics make them best suited to survive in their natural
environment reproduced more often than organisms that
were less well adapted. They then passed on those physical

Genetic Concepts Then and Now
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