Unisexual Crosses 255 



find their mates in the cross, so these two will 

 also meet one another. They will unite just as 

 well as though they were both active or both 

 dormant. For essentially they are the same, 

 only differing in their degree of activity. From 

 this we can infer, that in the crossing of varie- 

 ties, no unpaired remainder is left, all units 

 combining in pairs exactly as in ordinary fertil- 

 ization. 



Setting aside the contrast between activity 

 and latency in this single pair, the procedure in 

 the inter-crossing of varieties is the same as in 

 ordinary normal fertilization. 



Summarizing this discussion we may con- 

 clude that in normal fertilization and in the 

 inter-crossing of varieties all characters are 

 paired, while in crosses between elementary 

 species the differentiating marks are not mated. 



In order to distinguish these two great types 

 of fertilization we will use the term bisexual for 

 the one and unisexual for the other. The term 

 bisexual crosses then conveys the idea of com- 

 plete bisexuality, all unit-characters combining 

 in pairs. Unisexual crosses are those in which 

 one or more units do not find their mates and 

 therefore remain unpaired. This clear and 

 useful term was proposed by MacFarlane when 

 studying the minute structure of plant hybrids 

 in comparison with that of their parents (1892). 



