xiv HORMONES AND HEREDITY 



Whenever a changed environment acts upon the 

 organism, therefore, it to some extent affects the 

 normal excretions and secretions of some or all of 

 the various tissues, and these react not onlv on the 

 tissues themselves, but also to a less degree upon the 

 determinants representing them in the germ-plasm. 

 Thus the relative size of the brain has decreased in 

 the tame rabbit. This may be due to disuse ; the 

 excretions and secretions of the nervous tissues would 

 be diminished, and the corresponding determinants 

 less stimulated. Another instance is afforded by 

 pigmentation of the skin in man ; which varies with 

 the amount of light and heat from the sun to which 

 the skin is exposed. Specific excretory products of 

 pigment in the skin may stimulate the pigment 

 determinants in the germ-plasm to increased vigour. 

 But only those characters of which the correspond- 

 ing tissues possess a specific secretion or excretion 

 could become hereditary in this way. For instance, 

 the brawny arm of the blacksmith could not be 

 transmitted, as it is scarcely possible that the arm 

 muscles can have a secretion different from that of 

 the other muscles. 



In 1904, P. Schiefferdecker ^ made the definite 

 suggestion that the presence of specific internal 

 secretions could be very well used for the explanation 

 of the inheritance of acquired characters. When 

 particular parts of the body were changed, these 

 modifications must change the mixture of materials 

 in the blood by the substances secreted by the 

 changed parts. Thereby would be found a connexion 

 between the modified parts of the body and the 

 germ-cells, the only connexion in existence. It is to 



^ p. Schiefferdecker, Ueber Symbiose. S.B. d. Niederrhein. Gesellsch. 

 zu Bonn. Sitzung der Medicinischen Sektion, 13 Juni 1904. 



