PRESIDENTIAL ADDRESS. 31 



one or more of them during their lifetime, owing to some change 

 in the condition of their life. The first of these two groups is 

 of predominant importance, in respect to the number of well- 

 ascertained facts that it contains, many of which it is possible 

 to explain in a broad and general way, by more than one theory 

 based on the hypothesis of organic units. The second group 

 includes much of which the evidence is questionable or difficult 

 of verification and which, as I shall endeavor to show, does not 

 for the most part, justify the conclusion commonly derived from 

 it." He further says that his theory "is largely based on the 

 arguments and conditions brought forward by Mr. Darwin in 

 support of pangenesis ; nevertheless the conclusions in this 

 paper will be seen to differ essentially from his own. Pan- 

 genesis appears more especially framed to account for the cases 

 which fall in the second of the above-mentioned groups which 

 are of a less striking and assured character than those of the 

 first group, and it will be seen that I accept the theory of pan- 

 genesis with considerable modification, as a supplementary and 

 subordinate part of a complete theory of heredity, but by no 

 means for the primary and more important part." 



He employs the term stirp " in a special sense to express 

 the sum-total of the germs, gemmules, or whatever they may 

 be called, which are to be found, according to every theory of 

 organic units, in the newly fertilized ovum that is in its 

 earliest pre-embryonic stage." In defending the theory of 

 organic units he says : "We must bear in mind that the 

 alternative hypothesis of a general plastic force resembles that 

 of other mystic conceptions current in the early stages of 

 many branches of physical science, all of which yielded to 

 molecular views, as knowledge increased." 



The paper is an exceedingly luminous contribution to the 

 subject, and the theory advanced may be designated in general 



