Bisexual Crosses 307 



True elementary species differ from each 

 other by unit-characters. They have arisen 

 by progressive mutation. Such characters 

 are not contrasting. One species has one 

 kind of unit, another species has another kind. 

 On combining these, there can be no inter- 

 change. Mendelism assumes such an inter- 

 change between 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 unisexual characters lie side by 

 side with paired or bisexual qualities, and they 



