Darwin knew that many of his scientist colleagues were active freethinkers and were therefore considered political revolutionaries. Darwin himself was by nature anything but revolutionary and his caution in announcing the gradual evolution of species through natural selection was based on his distaste for controversy. He carefully avoided saying anything about the relationship between his findings and religious belief.

Freethinking scientists had no such hesitations. When ecclesiastics and political conservatives condemned Darwin's book, atheist and agnostic scientists welcomed battle. They saw themselves as armed with the evidence to defeat the conservative clerics. The popular press had its own twist. Although Origin of Species had said nothing about the descent of man, the implication that man came from a "lower" species such as the monkey led to caricatures of Darwin's bearded face engrafted on a monkey's body. The notion that man was descended from an apelike ancestor by chance variation, rather than divinely created to dominate all nature, revolted many.

Although 150 years later, the debate may have changed character, it still goes on. But we should pause to ask: Why has Darwin's theory of evolution become the foundation stone of biological science, no longer hypothesis but almost universally accepted by scientists as fact? Three kinds of observations continue to confirm Darwin's theory. The most familiar may be the elaboration of the fossil record of long extinct animals. Paleontologists have constructed long chains showing the descent of animal species and their gradual evolution over millions of years. Although there are gaps in the fossil record, discoveries of fossil creatures that help to fill in the gaps in Darwin's argument are still being found and are instantly noted in the media. A recent example is the unearthing in China of small feathered dinosaurs that afford a link to the origin of birds.

In the Galapagos Islands Darwin had realized that he was observing evolution in action when he saw the variation in finches and lizards from one island to the other. Now, we realize that we can perceive evolution by natural selection in simple creatures in time scales of only a few years. Because bacteria growing under favorable circumstances in the laboratory divide every few hours we can do a breeding experiment overnight. Put a strain of bacteria that require a particular sugar in a medium containing a different sugar. After a pause, the culture begins to grow the next day because a few variants that can use the new sugar have been selected. That is perhaps artificial, but we also observe natural selection of bacteria in the everyday environment. Widespread use of antibiotics has led to the appearance of mutant strains of bacteria untouched by commonly used antibacterials. We endeavor to fight these by ever developing new antibiotics. Introducing antibiotics created a pressure for disease organisms to develop a new way of surviving in the changed environment. Evolution by selecting mutations that improve survival is a way of life among bacteria.

New strains of influenza virus appear in the wild every year. Without these new strains of the virus, influenza would soon die out because the animals and man who have recovered from a strain of the virus have lifetime immunity against recurrent infection with that strain. The HIV virus appeared as if from nowhere in Africa less than a generation ago. We believe that it existed in apes for years unable to infect man, but eventually mutated into a form that can infect a host much more numerous than wild apes. Evolution of several living creatures has, therefore, gone on during our lifetime.

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