Unisexual Crosses 251 



For our present purposes we are to deal with 

 hybrids only in so far as they afford the means 

 of a still better distinction between elementary 

 species and varieties. I will try to show that 

 these two contrasting groups behave in quite a 

 different manner, when subjected to crossing 

 experiments, and that the hope is justified 

 that some day crosses may become the means of 

 deciding in any given instance, what is to be 

 called species, and what variety on physiolog- 

 ical grounds. It is readily granted that the 

 labor required for such experiments, is perhaps 

 too great for the results to be attained, but then 

 it may be possible to deduce rules from a small 

 series of experiments, which may lead us to a 

 decision in wider ranges of cases. 



To reach such a point of view it is necessary 

 to compare the evidence given by hybrids, with 

 the conclusions already attained by the com- 

 parison of the differentiating characteristics of 

 allied forms. 



On this ground we first have to inquire what 

 may be expected respecting the internal nature 

 and the outcome of the process of crossing in 

 the various cases cited in our former discussion. 



We must always distinguish the qualities, 

 which are the same in both parents, from those 

 that constitute the differentiating marks in 

 every single cross. In respect to the first 



