No. 526] SHORTER ARTICLES AND DISCUSSIONS 625 



early hybridists of plants, Gartner, Kolreuter, Nageli, Gordon 

 and others, were familiar with the frequent failure of pollen, 

 placed upon the stigma of certain plants, to produce seed in the 

 ovules of that plant, and with the fact that even if seed is ob- 

 tained, it may not grow when planted. Kolreuter and Gartner 

 saw in sterility the criterion wherehy species may be distin- 

 guished. If two forms bred together produced seed, and the 

 seed was capable of growing into perfect plants, the two forms 

 were considered as belonging to the same species. If, on the 

 other hand, no seed was produced, or if produced, this seed 

 was incapable of growth, the two forms were considered as be- 

 longing to distinct species. However, as Darwin points out, 

 these two workers were biased in their appreciation of sterility, 

 as a valid criterion for distinguishing species, for they themselves, 

 as have indeed all workers since them, described all degrees of 

 sterility under such circumstances, from slight deviations in form 

 of the hybrid plants, through n condition where the seed, although 

 formed, was shrivelled and incapable of producing the young 

 plant, to complete sterility, where no indication of even a pollen- 

 tube is seen in the style of the pistil. Obviously, the personal 

 equation of the experimenter will be a potent factor in deter- 

 mining whether there were two species, or a single one involved 

 in questionable cases, and the definiteness of the criterion is 

 largely detracted. To Darwin, who had carried out thousands 

 of crosses, the use of sterility as a distinguishing character for 

 species was an impossibility, and this fact was utilized by him in 

 combating the criticism urged against the theory of natural 

 selection, that it could not account for the origin of sterility by 

 assuming that it was a factor in isolating species. 



It has been only within comparatively recent times that an 

 understanding has been reached as to what are the intimate re- 

 sults of sterility. It is true that Gartner knew that the pollen 

 of many hybrids was shrivelled and functionless, but farther 

 than this, nothing was known of the condition of the spore- 

 bearing and gamete-bearing parts; for the role of the pollen- 

 tube, with the nuclei contained, and of the ovum in the ovary 

 was as yet unknown and the discovery of the alternation of gen- 

 erations in the higher plants had not been made. Not until 

 the reduction of the tetrads had ben worked out was it possible 



seed were produced, to understand why abnormalities occurred 

 in the young hybrid plants. 



