282 JENNINGS: CHANGES IN HEREDITARY CHARACTERS 



Can we say that they leave certain views admissible? Can we 

 go farther and say that they make certain views probable? 

 I shall hardly be so bold as even to ask whether they establish 

 any particular views, though even that has been at times affirmed. 



These questions have, of course, been raised thousands of 

 times; it is only because knowledge does advance, because 

 experimental work has been enormously multiplied of late, that 

 there is reason to bring them up anew. I am going to try to 

 put before you the present situation as it appears to me. 



What we may call the first phase of the modern experimental 

 study of variation is that which culminated in the establishment 

 of the fact that most of the heritable differences observed be- 

 tween closely related organisms — between the members of a 

 given species, for example — are not variations in the sense of 

 alterations; are not active changes in constitution, but are per- 

 manent diversities ; they are static, not dynamic. This discovery, 

 like that of Mendelian heredity, was, as you know, made long 

 ago by the Frenchman Jordan; but, as in the case of Mendelism, 

 science ignored it and pursued cheerfully its false path till the 

 facts were rediscovered in recent years. All thorough work 

 has led directly to this result : that any species or kind of organ- 

 ism is made up of a very great number of diverse stocks, differing 

 from each other in minute particulars, but the diversities in- 

 herited from generation to generation. This result has in recent 

 years dominated all work on the occurrence of variations; on 

 the effects of selection; on the method of evolution. The con- 

 dition is particularly striking in organisms reproducing from a 

 single parent, so that there is no mixing of stocks; I found it in 

 a high degree in organisms of this sort which I studied. Thus 

 the in usorian Paramecium I found to consist of a large number 

 of such heritably diverse stocks, each stock showing within itself 

 many variations that are not heritable. 2 Difflugia corona, 

 which I have recently been studying, shows the same condition 

 in a marked degree. 3 As you know, a host of workers have 

 found similar conditions in all sorts of organisms. It led to the 



2 See Jennings, 1908, 1909, 1910, 1911. (See Bibliography.) 



3 Jennings, 1916. (See Bibliography.) 



