Natural selection is one of the cornerstones of modern biology. The term was introduced by Darwin in his groundbreaking 1859 book "On the Origin of Species", in which natural selection was described by analogy to artificial selection, a process by which animals and plants with traits considered desirable by human breeders are systematically favored for reproduction. The concept of natural selection was originally developed in the absence of a valid theory of heredity; at the time of Darwin's writing, nothing was known of modern genetics. The union of traditional Darwinian evolution with subsequent discoveries in classical and molecular genetics is termed the "modern evolutionary synthesis". Natural selection remains the primary explanation for adaptive evolution.