Recent Advances in the Study of Heredity. 169 
according to the theory of ancestral contributions by the somatic 
characters of their parents and ancestors. And I submit that the 
words of Weldon’s which I have quoted, show that a belief in the 
latter theory has persisted at least as late as 1901, and that there 
is therefore some justification for the view that what I have called 
the third period begins, roughly, with the 20th century. 
I cannot better recall your attention to the tentativeness of 
this brief outline than by quoting part of the conclusion to the 
Zoonomia. “ What I have thus delivered I beg to be considered 
rather as observations and conjecture, than as things explained 
and demonstrated.” 
