14 JOUKNAL OF THE MlTCHELL SoCIETY [^July 



Similar experiments have been carried out involving two 

 and even three pairs of such characters, but the combina- 

 tions become too complex for presentation without figures. 

 Suffice it to say that with but few exceptions any pair of 

 characters will behave as above described. 



Cases such as those cited show that a character may 

 be extracted in a pure state from a hybrid even when it is 

 completely masked by a dominant. Characters must there- 

 fore be represented in the germ cells by independent units 

 that never lose their identity. 



Mendelism thus puts in the hands of the experimentalists 

 a definite standard of measurements by means of which he 

 may test the hereditary constitution of living organisms, and 

 to the practical breeder its gives a definite formula in ac- 

 cordance with which he may purge a chosen race of unde- 

 sirable characters and even supplant them with desirable 

 ones extracted from many sources, with somewhat the same 

 exactness that a chemist extracts and combines his chemicals. 



THE PUKE LINE 



Before great progress can be made in any science, the 

 fundamental units which enter into its facts must be clearly 

 understood. "Chemistry was alchemy until the chemical 

 elements were identified and isolated.' ' Similar fundamental 

 units in heredity are the Mendelian unit characters, and we 

 now speak of heredity in terms of unit characters rather than 

 of the individual as a whole. 



A species as the term is today coming to be understood 

 consists of subsidiary groups of individuals, which groups 

 differ from each other in average size, structure, color and 

 other unit characters, which in heredity, according to Jen- 

 nings, behave "as rigid as iron." The progeny of individuals 

 belonging to one such group constitutes the so-called "pure 

 line." This conception rests upon the brilliant and independ- 

 ent investigations of the Danish botanist Johanssen, the 

 American zoologist, Jennings, head of his department at the 

 Johns Hopkins T^niversity, and the Swedish botanist Kilsson. 



