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 



