Unisexual Crosses 253 



quirement of some new character. The deriva- 

 tive species has one unit more than the parent. 

 All other qualities are the same as in the par- 

 ent. Whenever such a derivative is sexually 

 combined with its parent the result will there- 

 fore be exactly as in a normal fertilization. In 

 such ordinary cases it is obvious that each char- 

 acter of the pollen-parent is combined with the 

 same character of the pistil-parent. There may 

 be slight individual differences, but each unit- 

 character will become opposed to, and united 

 with, the same unit-character in the other par- 

 ent. In the offspring the units will thus 

 be paired, each pair consisting of two equivalent 

 units. As to their character the units of each 

 single pair are the same, only they may exhibit 

 slight differences as to the degree of develop- 

 ment of this character. 



Now we may apply this conception to the sex- 

 ual combination of two different elementary 

 species, assuming one to be the derivative of 

 the other. The differentiating mark is only 

 present in one of the parents and wanting in the 

 other. While all other units are paired in the 

 hybrid, this one is not. It meets with no mate, 

 and must therefore remain unpaired. The 

 hybrid of two such elementary species is in 

 some way incomplete and unnatural. In the 

 ordinary course of things all individuals derive 



