44 BIOLOGICAL ASPECTS OF HUMAN PROBLEMS 



ity of certain animal qualities, and while it has not 

 been shown to apply to all the phenomena of heredity, 

 it has been a most welcome discovery to the biologist 

 because it affords something approaching a demon- 

 stration that a perfectly definite and apparently 

 simple machinery must underlie the transmission 

 of hereditary characters in general. Mendel's prin- 

 ciple cannot readily be stated in a brief definition 

 because it involves several distinct though closely 

 related elements. One is that hereditary characters 

 often show a remarkable degree of independence, 

 so that by means of crossing experiments they may 

 be combined and recombined in many ways without 

 permanently blending, almost as if they were repre- 

 sented by distinct material substances which may 

 be put together or taken apart like the blocks of a 

 building or the cards of a pack. It is therefore 

 possible to study heredity accurately by concentrat- 

 ing the attention upon only one or a few such "unit 

 characters. " A second principle is that in respect 

 to hereditary constitution the sexually produced 

 organism is of double or "duplex" composition, 

 owing to its origin from two parents (more accu- 

 rately from two germ cells). A third, and the one 

 which is the essence of "Mendel's Law," is that the 

 germ cells are not of duplex, but of "simplex," 

 composition. Thus, in respect to any particular 

 character, the germ cells produced by a hybrid are 

 not hybrid, but "pure" ; i.e. possess the capacity 

 to transmit but one of the two different characters 



