348 



NA TURE 



\August 10, 1882 



excellent instance of the same sort. It belongs to the 

 family of the Omtgracea, which are highly evolved poly- 

 petalous plants, with the petals reduced to four or two in 

 number, and placed above instead of below the ovary. 

 We should thus naturally expect them to be pink or lilac, 

 and this is actually the case with most of our native 

 species. Why, then, is the evening primrose yellow ? 

 Because it is a night-flowering plam, fragrant in the even- 

 ing, and its pale yellow colour makes it easily recognisable 

 by moths. In this case, however, two points mark it off 

 at once from the really primitive yellow flowers. In the 

 first place, it has not the bright golden petals of the but- 

 tercup, but is rather more of a primrose tint ; and this is 

 a common distinguishing trait of the later acquired 

 yellows. In the second place, it belongs to a genus in 

 which red and purple flowers are common, whereas the 

 buttercups are almost all yellow or whitey-yellow, and the 

 potentillas mostly yellow or white. In short, primitive 

 yellow flowers are usually golden, ar.d belong to mainly 



lower lip. In G. ochroleuca, the whole corolla has become 

 pure yellow. In this way, one can understand the occur- 

 rence of such a flower as Lamiian galeobdolon, especially 

 since an allied species, L. album (Fig. 23), is white, and 

 all the genus is extremely variable, in colour. Indeed, it 

 is to be noted that the yellow labiates do not commonly 

 occur among the less developed thymes, mints, and mar- 

 jorams, but among the extremely specialised Staehydea, 

 which have very modified flowers, and usually variegated 

 or spotted lips. They seem to be essentially reversionary 

 forms from purple or blue species, spotted with yellow. 



Another hint of Retrogression is given us by flowers 

 like our English balsams, Imputiens noli-me-tatigere and 

 /. fulva, in the fact that their yellow is generally dappled 

 with numerous spots of deeper colour. The balsams are 

 highly modified irregular Geraniacea, sepals and petals 

 being both coloured ; and at first sight it seems curious 

 that our species should be yellow, while the simpler Gera- 

 niums and Erodiums are pink or red. But the genus as 

 a whole contains many red and variegated species, and 

 alters in colour with much plasticity in the hands of gar- 

 deners. /. noli-me-tangere is pale yellow, spotted with 

 red ; I.fulva is orange, dappled with deep brown. Both 

 are almost certainly products of retrogressive selection. 



In the Primulacece, we find similar instances. Hottonia 

 paluslris, a less developed form, is rosy lilac. Cyclamen 

 curopcciim is white or rose-coloured. Trientalis curopaa 

 is white or pale pink, with a yellow ring. From such a 

 stage as this, it is easy to get at our primroses, cowslips, 



Fig. 2v — Corn bluebottle, briglv blue, highest type of Cynaroid composite 



yellow groups: reverted yellow flowers are often primrose 

 orange, or dull buff, and occur sporadically among blue, 

 red, or purple groups. 



There are other cases less immediately apparent than 

 these. For instance, Lamium galeobdolon, a common 

 English labiate, belonging to a usually purple or blue 

 family, is bright yellow. But we can form some idea of 

 how such changes take place if we look at the pansy, 

 which we have seen reason to believe is normally violet- 

 purple, but which usually has a yellow patch on the lowest 

 petal. In the pansy's var. lulea, the yellow extends over 

 the whole flower, no doubt because this incipient form has 

 succeeded in attracting some special insect, or else grows 

 in situations where yellow proves more conspicuous to 

 bees than blue or purple. So, again, another English 

 labiate, Galeopsis tctrahit, the hemp-nettle, has a pale 

 purple or white corolla, sometimes with a tinge of yellow 

 in the throat ; and in the var. versicolor, the yellow 

 spreads all over the flower, except a purple patch on the 



. ■ -' i ■ 



Fig 26. — Section of Ligulate Composite, all the florets retrogressionary 

 yeft w, 



and oxlips which have pale yellow corollas, with orange 

 spots at the throat. Indeed, one English species, Primula 

 farinosa, is pale-lilac, with a yellow centre : and this 

 might easily, under special circumstances, become pale 

 primrose all over. The cultivated varieties of the cowslip, 

 called Polyanthuses, readily assume various tints of 

 orange, red, and pink, always at the edge, the deep yellow 

 of the throat remaining unchanged. 



The colours of many Scrophularinca may be explained 

 in the same way. Perhaps the yellow of the mulleins is 

 primitive ; but as some species are white or purple, it is 

 just as likely to be retrogressive. In Linaria, we may 

 almost be sure that retrogression has taken place ; for we 

 can trace a regular gradation from lilac flowers with a 

 yellow palate, like /-. eymbalaria, to pale yellow flowers, 

 like L. vulgaris, which has the mass of the corolla prim- 

 rose, and t'ne palate orange (Fig. 24). Mimulus /ulcus is 

 also yellow, but it is usually marked inside with small purple 

 s^ots, and sometimes has a large pink or red patch upon 

 each lobe. In Melampyrum cris/atum, the yellow corolla 

 is variegated with purple : in M. prateiise, it has the lip 

 deeper in hue. All these genera include many purple and 

 variegated species ; and the yellow members almost 

 always bear evident marks of being descended from poly- 

 chromatic ancestors. 



The case of the yellow Composite^, especially the Ligu- 

 latiT, is more difficult to decide. It would seem as though 

 these plants, which have all their florets ligulate, must be 

 more highly developed than the Corymbifera:, which have 



