When scientists today work to decode the human genome, they use high-tech methods to view the microscopic chromosomes and even pluck individual genes out of a cell. But in Darwin's time, it was impossible to see any of that. No one was sure how animals or plants passed down traits. And Darwin knew that the lack of an explanation for heredity left a big gap in his theory of natural selection.
In one of the great triumphs of scientific experimentation, Austrian biologist and monk Johann Gregor Mendel, Darwin's contemporary, solved this problem in the mid-nineteenth century.
Ironically, Darwin never found out. The results of Mendel's carefully designed and meticulously executed experiments, which involved nearly 30,000 pea plants followed over eight generations, were ignored until long after both he and Darwin were dead.
Darwin, like many of his contemporaries, speculated that characteristics of the parents were blended -- like mixing paint -- as they passed to the offspring. But if that were true, some of Darwin's critics pointed out, then how could a single fortunate mutation be spread through a species? It would be blended out, just as a single drop of white paint would be in a gallon of black.
Mendel read Darwin with deep interest, but he disagreed with the blending notion, hypothesizing instead that traits, such as eye color or height or flower hues, were carried by tiny particles that were inherited whole in the next generation.
The patient monk carefully bred and cross-bred pea plants to see how a few specific traits -- height was one -- were passed down. When Mendel bred a tall plant to a short one, all of the offspring were always tall, never blending to medium size. When he then bred those offspring together, three out of four of their offspring were tall, but one was short.
Mendel knew exactly what this meant. Height was passed down in a particle we now call a gene (though Mendel never used that term himself). A plant was short or tall depending on the random combination of genes it inherited.
So an adaptive mutation could spread slowly through a species and never be blended out. Darwin's theory of natural selection, building on small mutations, could work. But no one at the time understood the implications of Mendel's experiments. He soon left biology to focus on running his monastery. Only in 1900 was his work rediscovered.
Only then did Mendel -- who had worked without a microscope, without computers, but with a thoughtful hypothesis, a carefully designed experiment, and enormous patience -- receive the credit for one of the great discoveries in the history of science.