Unisexual Crosses 249 



manner in which all advance in the animal and 

 vegetable kingdoms must have taken place ; con- 

 tinuously adding new characters to the already 

 existing number. Contrasted with this method 

 of growing differentiation, are the retrogres- 

 sive modifications, which simply retrace a 

 step, and the degressive changes in which 

 the backward step is taken up and old charac- 

 ters revived. No doubt both of these methods 

 have been operative, but they are evidently not 

 in the line of general advancement. 



In all of these directions we see that the dif- 

 ferentiating marks show more or less clearly 

 that they are built up of units. Allied forms 

 are separated from each other without interme- 

 diates. Transitions are wholly wanting, al- 

 though fallaciously apparent in some instances 

 owing to the wide range of fluctuating variabil- 

 ity of the forms concerned, or to the occurrence 

 of hybrids and subvarieties. 



These physiological units, which in the end 

 must be the basis for the distinction of the sys- 

 tematic units, may best be designated by the 

 term of ' ' unit-characters. ' ' Their internal na- 

 ture is as yet unknown to us, and we will not 

 now look into the theories, which have been pro- 

 pounded as to the probable material basis un- 

 derlying them. For our present purpose the 

 empirical evidence of the general occurrence of 



