88 Siiddoi Appearance and Constancy. 



I shall conclude by referring" to a race of MeliJotus 

 coerulea, the possession of which I owe to the kindness 

 of Prof. ]\I. W. Beyerinck (p. 63). Its leaflets^ are 

 fused to a single blade in which the three main nerves 

 still diverge from the base. The blade moreover has 

 three distinct tips, the depth of the indentations between 

 these being subject to considerable fluctuating variability. 

 Not rarely the three parts are only united from the base 

 to halfway up or less, and sometimes they are separated 

 almost to the base and in rare cases even entirely so. 

 All these forms may occur on the same plant. But there 

 was no reversion in my experimental sowings ; every 

 plant exhibited this monophylly to a greater or less ex- 

 tent. 



§ 9- STERILE VARIETIES. 



One of the greatest difficulties presented by the cur- 

 rent doctrine of selection lies, as I have pointed out more 

 than once in the first volume of this work, in the fact 

 that the gradual origin of species, which is presupposed 

 by it, has never been observed. In every case in which 

 observations have been made sufficiently close to the 

 origin of a new form, they indicate a sudden change. 

 We do not find those gradual transitions which the doc- 

 trine of selection would lead us to expect. The new form 

 may be highly variable, and in that way the limits be- 

 tween it and the parent species may sometimes overlap; 

 but, as I have already shown (Vol. I, § 25, p. 430) trans- 

 gressive variability of this kind only provides a morpho- 

 logical transitional series and not a genetic one. 



My object in the present chapter is to bring together 



' This form has been described by Wydler, Flora, i860, p. 56, 

 and occasionally since. 



