PLANT HYBRIDIZATION BEFORE MENDEL 191 



from all other plants. A plant can thus not be distinguished imme- 

 diately as being of hybrid origin. 



2. Since species-hybrids are frequently fertile, and the indi- 

 viduals of pure species not seldom infertile, the perfect or im- 

 perfect structure of the sex-organs is not decisive concerning the 

 nature of an organism. From the sterility of the male and female 

 organs, nothing can thus be summarily concluded regarding hy- 

 bridity, or from the fertility of the same regarding their pure 

 origin. 



3. Hybrids constitute a regular intermediate formation, since 

 they have inherited their characters from the two parental species 

 in almost equal measure. An extension beyond this occurs only in 

 a very limited and quite definite manner. 



Since the capacity for sexual reproduction becomes weakened, 

 and the vegetative activities especially aroused, he therefore con- 

 cludes : 



"We can hence take a plant into consideration as a hybrid, only when 

 its systematic characters can respond to these demands." (p. 300.) 



The total point of view regarding the hybrid, as Nageli con- 

 siders it to be, is even more definitely summarized in the next 

 succeeding sentence. 



"When it is a question of the hybrid nature of a plant, the first and 

 most important criterion is that it be a middle form between two definite 

 species. This requirement is often left out of consideration." 



And again : 



"For the correct estimation of hybrids, it is especially to be remem- 

 bered that the most constant and important characters hold most ex- 

 actly the mean between the parent species, and that, on the other hand, 

 a character can so much the more approach the one species, the more 

 unimportant it is." (4f, p. 300.) 



4. Between two forms there exists only one hybrid middle 

 form, indifferently whichever of the parental forms was used as 

 the pollen parent. On the other hand, the hybrid may form vari- 

 eties, which approach the parents in- an irregular manner. 



5. Hybrid fertilization takes place through foreign pollen, 

 when its own pollen is kept away from the stigma. 



6. Species-hybrids have, as a rule, either quite infertile or 

 weakened reproductive organs. In the latter case they form, on 

 self-fertilization, a limited number of viable seeds, and die out 

 after a few or several generations. 



