576 Species According to the Theory of Miitation. 



the transposition of an internal character; from being 

 latent it becomes active; from semi-latent, semi-active; 

 and so on. If new factors are becoming active for the 

 first time after having been latent through a shorter or 

 longer series of ancestors, we speak of progressive muta- 

 tions. If the active characters again become latent, the 

 process is a retrogressive one. In all other cases it is 

 degressive. 



The phenomena of hybridization find a ready ex- 

 planation in the principles derived, in the first part of this 

 volume, from our consideration of the origin of species 

 and varieties. There are tw^o main types of crosses, the 

 bi-sexual or ]\Iendelian and the uni-sexual. The former 

 conform to the laws of segregation, they lead to various 

 combinations of elementary characters, and thus can lead 

 to the origin of as many new races as the number of pos- 

 sible combinations indicates. These races are constant; 

 the hybrids, however, always exhibit segregation in the 

 formation of their sexual cells and sometimes even in the 

 formation of buds. The hybrids of uni-sexual crosses 

 on the other hand are constant ; so far as my experience 

 goes, they do not segregate. If they are fertile they are, 

 as a rule, as true from seed as their parents ; but they 

 may inherit the inconstancy of these (if, for instance, 

 one of these belonged to an eversporting variety) and 

 transmit it to their posterity. 



A strong body of facts, which have been given else- 

 ■\vhere lead to the conclusion that crosses follow Mendel's 

 laws if one of the two parents stands in the relation to the 

 other of having arisen from it by retrogressive or de- 

 gressive mutation. This means that the two parents of 

 the cross possess exactly the same internal elementary 

 characters; but that one or more of these occur in dif- 



