Embryology is "the
strongest single class of facts in favor of change of forms."
Darwin considered the process that
a fertilized egg goes through on the way to becoming a multibillion-celled adult
as crucial to understanding how basic anatomical parts of both animals and
plants change over evolutionary time. The new science of evolutionary
developmental biology, "evo devo," is revealing the truth behind
Darwin's assertion quoted above. Until recently, scientists could say
that forms do change, but they couldn't say exactly how, because they didn't know which genes were
essential for the development of any organism. They now know that, remarkable
as it may seem, all organisms on Earth, from microbes to man (a human embryo is
seen here), share a common "tool kit" of so-called master genes,
which govern how bodies and body parts form. When and which of these genes are
turned "on" or "off" during the embryonic stage
determines how each organism is built.