Dobzhansky opens with a critique of Shaikh Abdul Aziz bin Baz, the then Grand Mufti of Saudi Arabia, for holding a belief based on scripture that the Sun revolves around the Earth. Dobzhansky asserts that "it is ludicrous to mistake the Bible and the Koran for primers of natural science. They treat of matters even more important: the meaning of man and his relations to God." He then criticizes the early English antievolutionist Philip Henry Gosse – who had proposed that fossils were created in the places where they were found – for blasphemously implying that God is deceitful. As he had said in his earlier presidential address, "If the living world has not arisen from common ancestors by means of an evolutionary process, then the fundamental unity of living things is a hoax and their diversity is a joke." These two themes of the unity of living things and the diversity of life provide central themes for his essay. of Drosophilafruit flies on Hawaii. The green arrows show another example, that of a plant, Cyanea. Addressing the diversity of life on Earth, Dobzhansky asks whether God was joking when he created different species for different environments. This diversity becomes reasonable and understandable, however, if Creation takes place not by the whim of the Creator "but by evolution propelled by natural selection." He further illustrates this diversity from his own investigation of the widely diverse range of species of fruit flies in Hawaii. Either the Creator, "in a fit of absent mindedness," created many species of fruit flies in Hawaii, or the fruit flies that arrived on the islands, diversified to fill a wide range of vacant ecological niches. He illustrates the unity of living things using the molecular sequence of cytochrome C, which Emanuel Margoliash and Walter M. Fitch had shown to be similar in a wide range of species, including monkeys, tuna, kangaroos, and yeast. This unity is further illustrated by the similarity of the embryos of different species. Either God deliberately arranged things "to mislead sincere seekers of truth" or these similarities are the result of evolution. Dobzhansky concludes that scripture and science are two different things: "It is a blunder to mistake the Holy Scriptures for elementary textbooks of astronomy, geology, biology, and anthropology." One response to this paper was a paper by Stephen Dilley, "Nothing in biology makes sense except in light of theology?". This argued that Dobzhansky's arguments all "hinge upon sectarian claims about God’s nature, actions, purposes, or duties"—claims that in Dilley's view required more justification and appeared mutually incompatible.
The notion of the "light of evolution" came originally from the vitalist Jesuit priest Pierre Teilhard de Chardin, whom Dobzhansky much admired. In the last paragraph of the article, Dobzhansky quotes from de Chardin's 1955 The Phenomenon of Man: The phrase "nothing in biology makes sense except in the light of evolution" has come into common use by those opposing creationism or its variant called intelligent design. While the essay argues that Christianity and evolutionary biology are compatible, a position described as evolutionary creationism or theistic evolution, the phrase is also used by those who consider that "in biology" includes anthropology, and those who consider a creator to be unnecessary, such as Richard Dawkins who published The Selfish Gene just three years later.
