i 3 6 THEORIES OF HEREDITY 



degree hereditary, as they would be if the germ-plasm 



were continuous. 1 



Repair, and the renewal of lost parts in certain animals, 

 is also explained by the persistence of substances or cells 

 of the same kind as those which were the precursors 

 of the injured tissue or lost part ;— substances or cells 

 which would be ready to initiate development under 

 the stimulus provided by an injury. 



The simplicity and beauty of Professor Weismann's 

 hypothesis of heredity commends it to our favourable 

 attention, and demands a searching inquiry into the 

 evidence for the supposed transmission of acquired or 

 somatogenic characters. 



Into this inquiry it is impossible to enter on the 

 present occasion. I will only mention the various lines 

 of evidence which require investigation. The evidence 

 may be either Direct or Indirect. Direct proof would 

 be afforded if an undoubtedly somatogenic character 

 could be shown to have reappeared in the offspring 

 sufficiently often to prevent its explanation as a coin- 

 cidence. Thus, if mutilations, or the results of training, 

 exercise, or education (as apart from predisposition), or 

 acquired diseases (some diseases are certainly blastogenic) 

 reappeared in the offspring as the result of the operation 

 of heredity, the required proof would be afforded and 

 the hypothesis of the continuity of the germ-plasm would 

 collapse. Many diseases are due to living organisms 

 (' germs '), and when these reappear in the offspring the 

 result is clearly due to inoculation of the embryo or even 

 the germ-cell (as in the silkworm disease), and is not 

 therefore due to the operation of heredity. 



The present [in 1889] adverse position of the medical 

 faculty is in part due to want of discrimination between 

 blastogenic and somatogenic characters ; in part to the 

 fact that the evidence on which they rely was collected 

 when the transmission of somatogenic characters was 



1 See Professor Windle's interesting papers on Teratology, published 

 during the last few years [previous to the date at which this address was 

 read] in the Journal of Anatomy and Physiology, and the Proceedings of 

 the Birmingham Philosophical Society. 



