226 PLANT HYBRIDIZATION BEFORE MENDEL 



ber. This unproductiveness varies in different species up to sterility so 

 complete that not even an empty capsule is formed." (lb, p. 468.) 



"it is also notorious that not only the parent species, but the hybrids 

 raised from them, are more or less sterile, and that their pollen is often 

 in a more or less aborted condition. The degree of sterility of various 

 hybrids does not always strictly correspond with the degree of difficulty 

 in uniting the parent forms. When hybrids are capable of breeding 

 inter se, their descendants are more or less sterile, and they often be- 

 come still more sterile in the later generations." {ib., p. 469.) 



"with the majority of species, flowers fertilized with their own pollen 

 yield fewer, sometimes much fewer seeds, than those fertilized with 

 pollen from another individual or variety." (ib., p. 469.) 



As the result of his investigations regarding sterility of pollen, 

 Darwin was able to render at least one service, that of removing 

 the obsession which had so long afflicted the study of the hybrid 

 question, viz., the variety-species discussion. He says: 



"it can thus be shown that neither sterility nor fertility affords any 

 certain distinction between species and varieties. The evidence from this 

 source graduates away, and is doubtful in the same degree as is the 

 evidence derived from other constitutional and structural differences." 

 (la, 2:4.) 



The question of the chemical and cytological basis for sterility 

 or non-receptivity to pollen remains still in part a field for the 

 investigator. 



One of the most important questions from the present-day 

 viewpoint which Darwin investigated was that of heterosis, the 

 relative vigor of the first generation hybrids as compared with 

 that of their parents. The following allusions occur in the "Origin 

 of Species." 



Darwin comments on the fact that crosses between individuals 

 of the same species, where they differ to a certain extent, give 

 increased vigor and fertility, while close-fertilization long con- 

 tinued almost always leads to physical degeneracy, and re- 

 marks : 



"We know also that a cross between distinct individuals of the same 

 variety, and between distinct varieties, increases the number of the off- 

 spring, and certainly gives to them increased size and vigour." (la, 

 2 : 269.) 



Darwin thoroughly investigated, as is well known, the com- 

 parative relation of the offspring of crossed to those of selfed 

 plants with respect to vigor. 



"I have made so many experiments, and collected so many facts, show- 

 ing on the one hand that an occasional cross with a distinct individual 



