Avunciilary vs. Collateral Crossings 257 



difference will in most cases be too slight. Only when 

 two or three or more units have been added successively 

 to those already present, will we recognize an increase in 

 the degree of organization. 



The progress of every individual species can appar- 

 ently take different directions. In some genera there are 

 species so typical that they may be regarded as the com- 

 mon origin of the others. Where these are lacking it is 

 manifest that the systematic relations are still too incom- 

 pletely known to us, or that the given forms have died out. 

 Every species can therefore be compared with its own 

 ancestors or with other descendants of the same ancestors. 



This consideration leads us to the recognition of two 

 different types of relationship, and therewith also of two 

 groups of crossings between allied species, which have to 

 be kept absolutely apart. One of them we shall call the 

 avunculary, the other the collateral. In the first case we 

 cross a form with an "avunculus" or ancestor in the direct 

 line, in the latter case with one of its lateral relatives. 

 Obviously the first relation is very simple while the latter 

 is more complicated. 



Every character and every unit corresponding to it, 

 which in a crossing is present in one species and lacking 

 in the older one, forms a special point of difference. 

 Hence the simplest case is the one in which there is only 

 one such difference between the two parents of a cross. 

 But generally several of them exist. 



Now in such a cross, the differing factors evidently 

 do not find any antagonists in the sexual cells of the other 

 parent. When, during fertilization, the pronuclei unite 

 into a double nucleus, all the other units are present in 

 pairs. Not so the differing ones ; they lie unpaired in the 

 hybrid. 



