Chap. IX.] HYBEIDS AND MONGEELS COMPARED. 41 



contrast with that in the succeeding generations, is a 

 curious fact and deserves attention. For it bears on 

 the view which I have taken of one of the causes of 

 ordinary variability; namely, that the reproductive 

 system from being eminently sensitive to changed 

 conditions of life, fails under these circumstances to 

 perform its proper function of producing offspring 

 closely similar in all respects to the parent-form. "Now 

 hybrids in the first generation are descended from 

 species (excluding those long- cultivated) which have 

 not had their reproductive systems in any way affected, 

 and they are not variable ; but hybrids themselves have 

 their reproductive systems seriously affected, and their 

 descendants are highly variable. 



But to return to our comparison of mongrels and 

 hybrids : Gartner states that mongrels are more liable 

 than hybrids to revert to either parent-form ; but this, 

 if it be true, is certainly only a difference in degree. 

 Moreover, Gartner expressly states that hybrids from 

 long cultivated plants are more subject to reversion 

 than hybrids from species in their natural state; and 

 this probably explains the singular difference in the 

 results arrived at by different observers : thus Max 

 Wichura doubts whether hybrids ever revert to their 

 parent-forms, and he experimented on uncultivated 

 species of willows ; whilst Naudin, on the other hand, 

 insists in the strongest terms on the almost universal 

 tendency to reversion in hybrids, and he experimented 

 chiefly on cultivated plants. Gartner further states 

 that when any two species, although most closely allied 

 to each other, are crossed with a third species, the 

 hybrids are widely different from each other ; whereas 

 if two very distinct varieties of one species are crossed 



