2o8 PLANT HYBRIDIZATION BEFORE MENDEL 



fetched to think of limiting the boundary of a genus to species which 

 are capable of mutual fertilization." (p. 456.) 



"We may therefore assert the rule that the races of one and the same 

 species, or of very nearly related species, almost always are capable of 

 mutual fertilization without special difficulty." (p. 450.) 



Focke calls attention to the interesting fact that different 



families and different genera are very unlike in respect to their 



capacity for cross-fertilization. 



"In some families, individual genera show very great differences in 

 their tendency to and their capacity of hybridization." (p. 451.) 



However, "whether two species may be crossed with one another or not, 

 can only be determined with certainty through experiment." (p. 451.) 



Focke calls attention to the matter of ecological species in rela- 

 tion to crossing, that deserves much further investigation. 



"It appears to be difficult to cross plants with one another, which in- 

 habit very different zones, or very different habitats (water and dry 

 places). When it succeeds, the hybrids are sterile." (p. 453.) 



However, he calls attention to the fact that : 



"The origin of plants from the old or the new world, from the north- 

 ern or the southern hemisphere, forms in and for itself no hindrance to 

 crossing. Evergreens and deciduous, day-blooming and night-blooming 

 plants may often cross without difficulty." (p. 454.) 



"In some genera or groups of species, in which hybrids easily origi- 

 nate, there are individual species which appear to be more inclined than 

 others to enter into hybrid combinations." (p. 454.) 



Focke calls attention to the fact that hybrid formation between 

 two species does not always succeed equally easily in both direc- 

 tions, mentioning the case of Mirabilis jalapa X M. longifiora : 



"Many other cases are furnished by breeders of hybrids, in which 

 hybridization has succeeded in only one way. if, however, the experiments 

 have not been carried on frequently and in various places, and with 

 different individuals and races of the parents, one can draw no far- 

 reaching conclusions from the failure." (p. 455.) 



"it has not seldom been observed that two species are mutually able to 

 effectively pollinate each other, but that A produces more seeds with the 

 pollen of B, then B with the pollen of A." (p. 455.) 



According to Focke, 



". . . most of these cases come from Gartner's experience, and require 

 still further confirmation, if indeed the occurrence of this relation may 

 not be completely questioned." (p. 455.) 



Focke calls attention to an impression he had gained: 



"That genera with more or less zygomorphic flowers, which belong to 

 families in which actinomorphy prevails, show quite an especial inclina- 

 tion toward hybrid formation. 



"whoever is not able to recognize immediately from his own observa- 

 tion the fluidity and mutability of the series of living forms, a few 



