PLANT HYBRIDIZATION BEFORE MENDEL 225 



upon our knowledge of heredity, but in which likewise, until re- 

 cently, comparatively little experimental work had been done 

 since his time. As the result of his own studies, supplemented by 

 those of Hildebrand and Fritz Miiller, he was able to say: 



"We may therefore confidently assert, that a self-sterile plant can be 

 fertilized by the pollen of any one of a thousand or ten thousand in- 

 dividuals of the same species, but not by its own." {ib., p. 347.) 



Regarding all the causes of sterility, or inability to accept 

 fertilization, we are still somewhat at a loss for a complete ex- 

 planation, although recent chromosome discoveries are throwing 

 light upon the subject. Darwin states the situation in his time: 



"The veil of secrecy is as yet far from lifted ; nor will it be until we 

 can say why it is beneficial that the sexual elements should be differ- 

 entiated to a certain extent, and why, if the differentiation be carried 

 still further, injury follows. It is an extraordinary fact, that, with many 

 species, flowers fertilized with their own pollen are either absolutely or 

 in some degree sterile ; if fertilized with pollen from another flower on 

 the same plant, they are sometimes, though rarely, a little more fertile ; 

 if fertilized with pollen from another individual or variety of the same 

 species, they are fully fertile ; but if with pollen from a distant species 

 they are sterile in all possible degrees, until utter sterility is reached. 

 Thus we have a long series with absolute sterility at the two ends ; at 

 one end due to the sexual elements not having been differentiated, and 

 at the other end to their having been differentiated in too great a degree, 

 or in some peculiar manner." {ib., pp. 460-1.) 



The questions which Darwin raises in this connection are as fol- 

 lows (p. 458) : 



1. Why the individuals of some species profit greatly, others 

 very little, by being crossed. . 



2. Why the advantages from crossing seem to accrue exclu- 

 sively now to the vegetative and now to the reproductive 

 system, although generally to both. 



3. Why some members of a species should be sterile, while 

 others are entirely fertile with their own pollen. 



4. Why a change of environment or of climate should affect 

 the sterility of self-sterile specPes. 



5- Why the members of some species should be more fertile 

 with the pollen from another species than with their own. 



Regarding the general matter of sterility in hybrids, Darwin 

 comments as follows : 



"it is notorious that, when distinct species of plants are crossed, they 

 produce, with the rarest exceptions, fewer seeds than the normal num- 



