Appendix C. 177 



Oesler. Bot. Zeitschrift, XXI, 1871, where he alludes to 

 sundry other papers of his own advocating similar views.) 



The relation of these observations to Jordan's especes affinti 

 is obvious. We have only to suppose that some such slight 

 and constant difference characterizes the sexual elements of 

 these allied varieties as demonstrably characterizes their 

 morphology, and we can understand how pollen-prepotency 

 would keep the forms distinct such forms, therefore, being 

 so many records of such prepotency. 



Both from Kerner's work, and still more from that of 

 Jordan and Nageli, I conclude that (at all events in plants) 

 prepotency is the way in which physiological selection 

 chiefly acts. That is to say, sudden and extreme variations in 

 the way of sexual incompatibility are probably rare, as com- 

 pared with some degree of prepotency. According as this 

 degree is small or great so will be the amount of the 

 corresponding separation. This view would show that in 

 plants the principle of physiological selection is one of 

 immensely widespread influence, causing (on the same 

 areas) more or less permanent varieties much below specific 

 rank. And when we remember on how delicate a balance 

 of physiological conditions complete correspondency of pollen 

 to ovules depends, we may be prepared to expect that the 

 phenomenon of prepotency is not of uncommon occurrence. 



Self-fertilization and Variability. It occurred to Count 

 Berg Sagnitz that, if physiological selection is a true 

 principle in nature, vegetable species in which self- 

 fertilization obtains ought to be more rich in constant 

 varieties than are species in which cross-fertilization rules. 

 For, although even in the latter case physiological isolation 

 may occasionally arise, it cannot be of such habitual or 

 constant occurrence as it must be in the former case. 

 Acting on this idea, Count Berg Sagnitz applied himself to 

 ascertain whether there is any general correlation between the 



III. N 



