stout: pollinations in cichorium intybus 417 



the search for the source of ^•ariations which might be the material 

 for selective processes of evolution and the obvious fact that 

 crossing as contrasted with selling was considered to lead to greater 

 variability made him the more willing to emphasize any evidence 

 that the former process is beneficial in itself even independently 

 of its relation to the increase of variability. 



It is always to be remembered that even though self-fertilization 

 is productive of more numerous and vigorous offspring, the 

 increased variability resulting from crossing might make the latter 

 a vastly more significant process from the standpoint of evolu- 

 tionary progress. A study of Darwin's data shows clearly that 

 the evil effects of self-fertilization in plants are not sufficiently in 

 evidence to establish the point. In fact in the two cases {Ipomoea 

 and Mimulus) continued longest in self-fertilization (ten genera- 

 tions) strains highly self-fertile and vigorous were maintained. 

 Darwin fully admits not only that the supposed evil effects of 

 inbreeding are scarcely apparent, but that "it is difficult to avoid 

 the suspicion that self-fertilization is in some respects advanta- 

 geous" ('77, p. 352). Darwin's whole contention on this point 

 rests on the comparative increase in vigor and fertility which ap- 

 peared in certain cases of crossing. Here, however, the results 

 are far from consistent and convincing. Nearly half the cases 

 considered show no such benefit. This was realized by Darwin 

 who interpreted such negative results as due to the similarity of 

 the crossed plants because of "having been self-fertilized and culti- 

 vated under nearly uniform conditions for several generations" 

 ('77, p. 281). 



It is not always clear just what type of sterility is involved in 

 Darwin's results. It is not fully evident that such differences as 

 did appear between self-fed and cross-fed stocks are due to impo- 

 tence as I have used the term. Darwin seems to ascribe cases of 

 lower fertility in selfing to a combination of a weakened condition 

 (a type of impotence) and a lack of differentiation in sex elements 

 (a physiological incompatibility), which, however, he is careful 

 to distinguish, as to its immediate cause at least, from complete 

 self-sterility or, as I am calling it, well-developed self-incompati- 

 bility. At least in two genera, Eschscholtzia and Reseda, Darwin's 

 data involve sterility due to physiological incompatibility. It is 

 most significant that when self-fertility does occur in Eschscholtzia 

 28 



