Unbalanced 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 

 a backward step is retraced and old characters 

 revived. No doubt both of these methods have 

 been operative on a large scale, but they are evi- 

 dently 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 hvbrids and subvarieties. 



These physiologic 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 



