These views, which have grown out of Lamarck’s fundamental ideas (“Philosophie zoologique,” 1809) are now usually associated with the theory advanced chiefly by Etienne Geoffrey St. Hilaire (“Philosophie zoologique,” 1830), the opponent of Cuvier, and the ally of Goethe, of the direct influence of the _monde ambiant_. The “surrounding world,” the influences of climate, of locality, of the weather, of nutrition, of temperature, of the salinity of the water, of the moisture in the air, and all other conditions of existence, influence the living organism. And they do so not indirectly, as is implied in the process of selection, simply playing the part of a sieve, and not themselves moulding and transforming, but _directly_ by necessitating the production of new developments in the living substance, new chemical and physiological activities, new groupings and changes of form, and new organs.
Darwin himself did not regard these two theories as opposed to the theory of selection, but utilised them as subsidiary interpretations. It is obvious, however, that at bottom they conceal an essentially different fundamental idea, which, if followed out to its logical consequences, reduces the “struggle for existence” to at most a wholly indifferent accessory circumstance. Weismann felt this, and hence his entirely consistent endeavours to show by great examples, such as the origin of flowers, the mutual adaptations of flowers and insects, the phenomena of mimicry, and many other cases, that neither the Lamarckian nor any other factor in evolution, except only natural, passive selection, suffices as an interpretation. From the Darwinian standpoint he is absolutely right, and must needs speak of the “omnipotence of natural selection,” for it must either be omnipotent, or it must give place to the other two factors, and retain only the significance we attributed to it in another connection (p. 157), which amounts to saying none at all. It is obvious enough why the discussion as to these factors should centre round the question of the “inheritance of acquired characters,” “acquired” either through the use or disuse of organs, the exercise or non-exercise of functions, or through the stimuli of the external world.