344 Adaptation to Environment 



Lewis has shown that if the optic cup is transplanted 

 tinder 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 exist- 

 ence 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 dis- 

 harmonies 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. 



The following observation may serve to give an idea 

 how small is the number of existing or durable forms 

 compared with the number of forms incapable of exist- 

 ence. We have mentioned the fact observed by Moenk- 

 house, the writer, and Newman, that it is possible to 

 fertilize the eggs of each marine bony fish with the 



