Progressive, Retrogressive, Degressive Mutations. z>17 



ferent conditions in the two parents; as for instance in 

 the union of latent characters with active, of semi-latent 

 with semi-active, and so forth. The opposite visible qual- 

 ities, determined by the two different conditions of the 

 same internal factor, constitute a so-called pair of char- 

 acters. Therefore, racial forms which differ from one 

 another by such qualities only, constitute a group for 

 themselves, in the theory both of hybridization and of the 

 origin of species. 



Uni-sexual crosses are of an entirely different nature 

 and lead to the direct production of constant hybrid 

 races. They occur when at least one character is present 

 in one parent, but is absent from the other. Hence Mac- 

 farlane's name uni-sexual. In more complicated cases 

 one form may have a larger excess of factors; or again, 

 each of the parents may possess factors which are absent 

 from the other. The excess will be unilateral, if one of 

 the forms has arisen directly from the other, but bi-lateral 

 if both have arisen in diverging directions from the same 

 ancestor. In tliese cases vicarious pairs of characters do 

 not exist in the two forms crossed, although externally 

 they may seem to do so. 



Therefore uni-sexual crosses correspond to progres- 

 sive mutations; the Mendelian law, however, to the retro- 

 gressive and degressive forms of differentiation. Con- 

 versely we may infer that characters which, when crossed, 

 behave in a Mendelian way, are in a latent or semi-latent 

 condition, and that in the uni-sexual crosses an internal 

 factor is entirely absent on one side. I willingly achnit 

 that the main purpose of this discussion is t(^ make my 

 principles clear, and to show how the two great branches 

 of the theory of mutation mav. in s]')ite of the \ast differ- 

 ence in their points of departure, ultimately lead t(^ the 



