The wonderful adaptations exhibited in the mating instincts seem to be due to definite substances secreted by the sex glands, as was shown by Steinach (Chapter VII). Here, again, the process as popularly conceived, is the reverse of the truth; those survive that have the equipment,--they did not acquire the equipment under the influence of environment. It is absolutely imperative for green plants that their stems and leaves be exposed to the light since only in this way are they able to form carbohydrates; and it is equally essential that the roots should grow into the soil so that the plant may get the nitrates and phosphates required to build up its proteins and nucleins. This result is, in the language of adaptationists, brought about by an adaptive response of the plant to the light. In reality this adaptive response is due (Chapter X) to the presence of a photosensitive substance present in almost all green plants. Lewis has shown that if the optic cup is transplanted under the skin of a young larva into any part of the body the skin in contact with the optic cup will form a lens; it looks as if a chemical substance from the optic cup were responsible for the formation of the lens. These examples might be multiplied indefinitely. They all indicate that apparent morphological and instinctive adaptations are merely caused by chemical substances formed in the organism and that there is no reason for postulating the inheritance of acquired characters. We must not forget that there are just as many cases where chemical substances circulating in the body lead to indifferent or harmful results. As an example of the first type, we may mention the existence of heliotropism in animals living in the dark, of the latter type, the inheritance of deficiencies like colour-blindness or glaucoma. While it is possible for forms with moderate disharmonies to survive, those with gross disharmonies cannot exist and we are not reminded of their possible existence. As a consequence the cases of apparent adaptation prevail in nature.