418 Origin of Each Species Considered Separately. 



acter. It certainly is in the Oenotheras. The older 

 species 0. biennis, 0. muricata and 0. Lamarckiana al- 

 ways produce, so far as we know, a pollen of which 

 some part, often as much as one-third, consists of sterile 

 grains. It would be very useful if some one would de- 

 termine the degree of this fertility; it would without 

 doubt follow Quetelet's law of fluctuating variability 

 and would probably exhibit also partial variability in a 

 high degree — since the percentage of sterile grains would 

 be likely to be high on weak lateral branches.^ Sterile 

 or almost sterile individuals may therefore appear from 

 time to time. For example I once found a plant of Oeno- 

 thera gig as, which, in spite of repeated attempts to fer- 

 tilize it with its own pollen, set no seed. And Oenothera 

 hrevistylis is much more often than not absolutely sterile, 

 in spite of the full development of its pollen; and this 

 sterility is closely correlated with the individual varia- 

 bility in the size of the fruits. 



It may happen that a new species absolutely lacks 

 pollen, as we have seen to be the case with O. lata. But 

 it does not follow from that, that every new form that 

 arises, if we find it first as a sterite plant, must, when 

 it arises once more, be sterile again. 



The incipient species in my cultures were as a rule 

 represented by solitary individuals. In rarer cases the 

 new form was represented in the same crop by two or 

 three seedlings; or was repeated in succeeding years. If 

 nothing more than rosettes of radical leaves were pro- 

 duced, absolute certainty as to the identity of the type 

 was of course out of the question ; but it is always better 

 ni cases like these to unite those which apparently belong 



^ See amongst others A. Jencic, Untcrsuchiingcn i'lher den Pol- 

 len hybridcr Pftanzen, Oesterr. Bot. Zeitschr., T. 50, 1900. 



