Balanced Crosses 807 



True elementary species differ from each 

 other in a number of unit-characters, which do 

 not contrast. They have arisen by progressive 

 mutation. One species has one kind of unit, 

 another species has another kind. On com- 

 bining these, there can be no interchange. 

 Mendelism assumes such an interchange be- 

 tween units of the same character, but in a 

 different condition. Activity and latency are 

 such conditions, and therefore Mendel's law 

 obviously applies to them. They require pairs 

 of antagonistic qualities, and have no connec- 

 tion whatever with those qualities which do not 

 find an opponent in the other parent. Now, 

 only pure varieties afford such pure conditions. 

 When undergoing further modifications, some 

 of them may be in the progressive line and 

 others in the retrogressive. Progressive modi- 

 fications give new units, which are not in con- 

 trast with any other, retrograde changes turn 

 active units into the latent condition and so give 

 rise to pairs. Ordinary species generally 

 originate in this way, and hence differ from 

 each other partly in specific, partly in varietal 

 characters. As to the first, they give in their 

 hybrids stable peculiarities, while as to the 

 latter, they split up according to Mendel's law. 



Unpaired or unbalanced characters lie side by 

 side with paired or balanced qualities, and they 



