Abstract
We celebrate this year the sesquicentennial anniversary of the publication of On the Origin of Species (1), one of the most important books ever written. The two great themes of The Origin are descent, with modification, of diverse species from common ancestors, and natural selection, which Darwin proposed as the chief agent of modification. He remarked, in Chapter VI, that “it is generally acknowledged that all organic beings have been formed on two great laws—Unity of Type, and the Conditions of Existence. By unity of type is meant that fundamental agreement in structure, which we see in organic beings of the same class, and which is quite independent of their habits of life. On my theory, unity of type is explained by unity of descent. The expression of conditions of existence…is fully embraced by the principle of natural selection.” The two great laws are conjoined, he noted, because natural selection will have adapted the parts of each being “during long-past periods of time,” so that “the law of the Conditions of Existence is the higher law; as it includes, through the inheritance of former adaptations, that of Unity of Type” (ref. 1, p. 168).
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