FACTS AND FINDINGS 31 



Mendel. Mendel's work was actually done in a preceding 

 generation, but as it remained unknown until our own, 

 and as it has exercised the profoundest influence on all 

 biological work since 1900, it deserves a place in this 

 survey. 



Mendel pointed out that definite numerical results were 

 to be obtained in crossing peas, which he explained by 

 two simple hypotheses — that of segregation and that of 

 independent assortment. 



The first of these is now accepted as a ''law" having 

 a very wide application. All later discoveries in the field 

 of heredity and genetics rest upon that law. It is, as 

 Morgan says, "a broad generalization based on numerical 

 data, and allows us to predict with accuracy the outcome 

 of a given solution." 



Practically all those biologists whose opinion was asked, 

 were unanimous in putting the experimental work on 

 genetics and heredity in the foremost rank of this genera- 

 tion's achievement. 



Mendel's second hypothesis applied only within certain 

 limits that can be accurately stated, but both hypotheses 

 hold very definitely for the cases he studied and for char- 

 acters of the majority of plants and animals studied since 

 then. However, much knowledge has been added in the 

 way of additional facts, so that we now speak of a "link- 

 ing" and a "crossing over" of certain characters, and with 

 such equipment added, we apply the laws over a much 

 wider field than Mendel did or could. 



Much of the difficulty of early workers in any new field 



