656 Geological Periods of Mutation. 
If now we follow this stem downwards, we must ob- 
viously sooner or later arrive at another mutation period, 
and of course one of which, although direct observation 
is no longer possible, so many products remain, that we 
may conclude with a high degree of probability its simi- 
larity with the period observed by me. I am referring 
to the differentiation of the sub-genus Onagra, and of its 
numerous species such as 0. biemiis L., O. miiricata L.. 
O. cruciata Nntt, etc. I have already dealt at length in 
the first volume (p. 440) with this hypothetical period, 
and therefore may now limit myself to representing this 
Onagra-pmod in Fig. 148 in the same way as the group 
above it, which relates to the variations now being pro- 
duced by Oenothera Lamarckiana. 
Obviously we may now continue our scheme down- 
wards. We next reach the sub-genus Euoenothera, many 
of the species of which are very like those of Onagra, and 
have, indeed, sometimes been confused with them, as, e. g., 
0. odorata with 0. suaveolens. From these we attain to 
the genus Oenothera itself, whilst other sub-genera form 
lateral branches, of which Knciffia (Fig. 89, p. 458) and 
Xylopleurum have been selected as examples in Fig. 149. 
I have so often made reference to the vestiges, left 
by other past, but relatively recent, periods of mutation 1 
that I may now confine myself to mentioning the follow- 
ing: Draba verna, according to JORDAN and ROSEN (Vol. 
1, p. 173 and Fig. 3 on page 22) ; Viola tricolor (Vol. I, 
p. 23, Fig. 4) according to WITTROCK'S researches, Hie- 
ra'cium, Rnbns, Rosa, Helianthemum and many other 
genera with their numerous closely related species are 
*It would appear, from WASMANN'S beautiful investigations, 
that certain beetles (of the genus Dinarda) which live in association 
with ants are at present in a mutable state. Biolog. Centralblatt, XXI, 
Nos. 22 and 23, Dec. 1901. 
