I 90 COOK 



and presented at first hand. The only regret is that the lecture 

 was not continued to the point of explaining how these facts are 

 to be interpreted and brought into relation with the more general 

 subjects of heredity and evolution. Details of scientific experi- 

 ments are interesting to specialists in the same field, but we do 

 not expect the general public, or even our colleagues in other 

 departments of science, to pause and give special heed to what we 

 have found unless we have reason to believe that our facts have 

 some particular and unusual significance. It is not improper 

 to ask, therefore, what are the new facts of general import which 

 have been revealed by experiments in Mendelism? 



Phenomena of this kind have received a large amount of 

 special study in recent years, because it has been claimed that 

 they prove new principles of heredity and open new routes to 

 the solution of the larger problems of organic development. 

 Such claims of great and special significance for Mendelism 

 are due largely to the fact that these experiments brought for 

 the first time a fairly definite mathematical factor into the prob- 

 lem of descent. In Mendelian crosses or hybrids there is a 

 definite and uniform proportion between the expression of char- 

 acters in what are called the first and second generations. It 

 has not unnaturally been supposed that this regularity of pro- 

 portion must obey an internal law or principle of descent govern- 

 ing the relations and combinations of characters. Definite 

 mathematical relations must represent, it has been argued, defi- 

 nite entities inside the germ-cells. Here, at last, appeared to be 

 a triumphant justification for the mechanical speculations of 

 Darwin, Nageli, and Weismann, to the effect that characters are 

 transmitted from generation to generation by means of minute 

 determinant particles or character-units of the germ-cells. It 

 was found possible to explain the mathematical relations of 

 typical cases of Mendelism by supposing that the presence or 

 absence of certain particles in the germ-cells determined the 

 presence or absence of the character in the adult organism. 



In a Mendelian cross the parents differ in at least one pair of 

 definitely contrasted characters. All the individuals of the so- 

 called first generation show the character of one of the parents, 

 which is called the dominant. In the following generations 



