On this date, the Danish botanist and geneticist Wilhelm Ludvig Johannsen was born. His experiments in plant heredity offered strong support to the mutation theory of the Dutch botanist Hugo de Vries (that changes in heredity come about through sudden, discrete changes of the heredity units in germ cells). Many geneticists thought Johannsen’s ideas dealt a severe blow to Charles Darwin’s theory that new species were produced by the slow process of natural selection.
Johannsen explained his ideas in his book entitled On Heredity and Variation (1896), which he revised and lengthened with the rediscovery of Gregor Mendel’s laws and reissued as The Elements of Heredity in 1905. The enlarged German edition of this work was published in 1909 and became the most influential book on genetics in Europe. In it, Johannsen coined the terms “gene”, “phenotype”, and “genotype”.