80 HYBEID VEEBASCUMS. Chap. H 



cautious in inferring the specific identity of two forms 

 from the presence of intermediate gradations ; nor 

 would it be easy in the many cases in which hybrids are 

 moderately fertile to detect a slight degree of sterility in 

 such plants growing in a state of nature and liable to be 

 fertilised by either parent-species. Thirdly and lastly, 

 these hybrids offer an excellent illustration of a state- 

 ment made by that admirable observer Gartner, namely, 

 that although plants which can be crossed with ease 

 generally produce fairly fertile offspring, yet well- 

 pronounced exceptions to this rule occur ; and here we 

 have two species of Verbascum which evidently cross 

 with the greatest ease, but produce hybrids which are 

 excessively sterile. 



