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. biennis L., O. muricata 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 

 0;/a^;'a-period in Fig. 148 in the same way as the group 

 above it, which relates to the variations now being pro- 

 duced by Oenothera Lamarchiana. 



Obviously we may now continue our scheme down- 

 wards. We next reach the sub-genus Enoenothera, 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. snaz'eolens. From these we attain to 

 the genus Oenothera itself, whilst other sub-genera form 

 lateral branches, of which Kneiffia (Fig. 89, p. 458) and 

 Xyloplenrnm 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^ 

 that I may now confine myself to mentioning the follow- 

 ing: Draha 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- 

 racium, Rnhus, Rosa, Helianthemwn 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 Dinar da) which Hve in association 

 with ants are at present in a mutable state. Biolog. Centralblatt, XXI, 

 Nos. 22 and 23, Dec. 1901. 



