FACTS OF INHERITANCE 163 



organ of the body can react upon the germ-cells in 

 a manner so specific that these can, when they 

 develop, reproduce the particular parental modifi- 

 cation or any approach to it. Darwin and Spencer 

 both faced this difficulty, and tried to meet it ; but 

 no one now accepts their provisional hypotheses. 

 It is true that a mechanism may exist though it 

 remains unknown ; it is true that important in- 

 fluences, mysterious in their nexus, pass from 

 reproductive organs to body; but we should not 

 have recourse to difficult hypotheses before we 

 are sure that there is any need for them. There 

 is no doubt that the germ-plasm may be influ- 

 enced by the blood, but this is different from 

 admitting the transmission of a f articular acquired 

 character. 



In a well-known case, where the evidence points, 

 according to some, to the heritability of an arti- 

 ficially induced epileptic condition, it has been 

 suggested that the epilepsy produces a toxin which 

 passes to the germ-cells so that the offspring are 

 epileptically affected. Now if we dared to suppose 

 that a deeply saturating modification produces a 

 representative chemical substance analogous to a 

 toxin, and that this passes to the germ-cells, the 

 hereditary reappearance of a modification would 

 be more conceivable. 



There are many who think that, sooner or later, 

 there must be a return to Darwin's idea of pan- 

 genesis — of specific substances passing from body 

 to germ-cells. The study of hormones is a line of 

 investigation that is of much interest in this con- 

 nection. " Hormones " are specific substances 

 produced by ceUs, and passed into the blood- 

 stream to play an important part in stimulating 



