I. Haeckel and His Opponents

Through the generalisation of these and similar results, Weismann comes to the conclusion: “The external influence is never the real cause of the difference, but plays the part of the stimulus, which decides which of the available dispositions shall come to development. The real cause, however, always lies in preformed changes of the body itself, and these—since they are constantly purposeful—can be referred in their development only to processes of selection,” to the selection of the fittest in the struggle for existence. The struggle for existence (selection) “alone is the guiding and leading principle in the development of the world of organisms” ( Aüssere Einflüsse der Entwickelungsreize, p. 49). The English investigators Francis Galton and Alfred Russel Wallace hold the same view as Weismann as to the non-inheritance of acquired characteristics and the omnipotence of selection.