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Corresponding to the same complex of empirical data, there may be

several theories, which differ from one another to a considerable

extent. But as regards the deductions from the theories which are

capable of being tested, the agreement between the theories may

be so complete that it becomes difficult to find any deductions

in which the two theories differ from each other. As an example,

a case of general interest is available in the province of

biology, in the Darwinian theory of the development of species by

selection in the struggle for existence, and in the theory of

development which is based on the hypothesis of the hereditary

transmission of acquired characters.