342 MEMOIRS OF THE NEW YORK BOTANICAL GARDEN 



cross-fertilization or, in the case of dimorphic and trimorphic 

 plants, from legitimate pollination (pollination usually involving 

 pistils and stamens of the same length). The causes operating 

 in such cases are considered to be different from those causing 

 self-sterility as seen in plants like Eschscholtzia, for Darwin states 

 that self-sterility (sterility due to physiological self-incompati- 

 bility) "must be different, at least to a certain extent, from that 

 which determined the difference in height, vigor, and fertility of 

 the seedlings raised from self-fertilized and crossed seeds; for 

 we have already seen that the two classes of cases do not by any 

 means run parallel" but that "this want of parallelism would be 

 intelligible if it could be shown that self-sterility depended solely 

 on the incapacity of the pollen tubes to penetrate the stigma of 

 the same flower deeply enough to reach the ovules" ('77, p. 342). 



Darwin fully realized that the effects of inbreeding, as judged 

 by vegetative vigor and fertilit3^ exhibit wide fluctuation, and that 

 his results show that decreased fertility does not always result 

 from continued self-fertilization or inbreeding in the cases of his 

 highly self- fertile strains of Ipomoea and Mimulus ('77) and in 

 the appearance of equal-styled and highly self-fertile varieties 

 of Primula veris and P. sinensis, so that, as he admits, it is 

 "difficult to avoid the suspicion that self-fertilization is in some 

 respects advantageous" ('77, p. 352). Burck ('08) very ex- 

 haustively reviews Darwin's results and raises the question 

 whether inbreeding ever decreases fertility and concludes that the 

 data show (a) that the greater fertility which appeared in crosses 

 involved impure varieties whose fertility has been already de- 

 creased by hybridization ; ib) that continued self-fertilization does 

 not lead to decreased fertility and (c) that pure (homozygous) 

 plants show no advantage when crossed either in vegetative \'igor 

 or fertility. 



id) Darwin's conception of the nature of the differentiation 

 involved in sexual rei)roduction, in relation especially to sterility 

 and fertility, emphasized and clearly delimited the current theory 

 of sex, that it is differences between the sexes and sex elements 

 that make sexual rei:>roduction possible and fruitful. Thus he 

 was of the opinion that "some degree of differentiation in the 

 sexual system is necessary for the full fertility' of the parent plants 

 and for the full vigor of their ()ffsi)ring" ('77, p. 344, 345). "Fer- 



