Editor's Note: This post is also appearing at the American Institute for Biological Sciences' Year of Science 2009: Celebrate Evolution. For more, see our tribute to Darwin on his 200th birthday.

February 12, 2009, marked the 200th birthday of two giants of the 19th century: Charles Darwin and Abraham Lincoln.  But the two great men are linked by something other than the simultaneity of their births.

Before we get to the details of that linkage, let me introduce a third larger-than-life character: Ernst Mayr.  In 2000, I had the unique privilege of working with one of the foremost evolutionary biologists of the 20th century, Ernst Mayr, on an essay he wrote for that year’s July issue of Scientific American, titled “Darwin’s Influence on Modern Thought.” 

Mayr, 95 years old at the time, still published regularly—he would write two more books before his death at 100—and had a wry sense of humor.  When I told him that my favorite work of his was One Long Argument: Charles Darwin and the Genesis of Modern Evolutionary Thought, because it was so straightforward, he replied, “Do you know that my books are so straightforward that the New York Review of Books has never reviewed one?”

Here is a key section of Mayr’s Scientific American article:

“Darwin refutes typology. From the time of the Pythagoreans and Plato, the general concept of the diversity of the world emphasized its invariance and stability. This viewpoint is called typology, or essentialism. The seeming variety, it was said, consisted of a limited number of natural kinds (essences or types), each one forming a class. The members of each class were thought to be identical, constant, and sharply separated from the members of other essences.

“Variation, in contrast, is nonessential and accidental. A triangle illustrates essentialism: all triangles have the same fundamental characteristics and are sharply delimited against quadrangles or any other geometric figures. An intermediate between a triangle and a quadrangle is inconceivable. Typological thinking, therefore, is unable to accommodate variation and gives rise to a misleading conception of human races. For the typologist, Caucasians, Africans, Asians or Inuits are types that conspicuously differ from other human ethnic groups. This mode of thinking leads to racism. (Although the ignorant misapplication of evolutionary theory known as “social Darwinism” often gets blamed for justifications of racism, adherence to the disproved essentialism preceding Darwin in fact can lead to a racist viewpoint.)

“Darwin completely rejected typological thinking and introduced instead the entirely different concept now called population thinking. All groupings of living organisms, including humanity, are populations that consist of uniquely different individuals. No two of the six billion humans are the same. Populations vary not by their essences but only by mean statistical differences. By rejecting the constancy of populations, Darwin helped to introduce history into scientific thinking and to promote a distinctly new approach to explanatory interpretation in science.”

At the time of the publication of Darwin’s Origin of Species in 1859, however, the legal status of African Americans in the soon-to-be-torn-asunder United States reflected a deeply typological viewpoint.  The Constitution had institutionalized this thinking mathematically: it considered slaves to be three-fifths of a full human being. 

Just over three years later, however, Lincoln swept such typological calculation away.  The Emancipation Proclamation, followed by the ratification of the Thirteenth Amendment to the Constitution, recognized in law what Darwin had made scientific: that all people were equally human, their individual characteristics just variations on the theme of humanity.

Lincoln at his last photographic sitting, 1865, via Wikimedia