88 Sudden Appearance and Constancy. 
I shall conclude by referring to a race of Mclilotus 
coerulca, the possession of which I owe to the kindness 
of Prof. M. W. BEYERINCK (p. 63). Its leaflets 1 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 
1 This form has been described by WYDLER, Flora, 1860, p. 56, 
and occasionally since. 
