August 10, 1882] 



NATURE 



547 



ferent hues than any other forms of flowers in the whole 

 world. 



On the other hand, primitive yellow flowers of the 

 earliest type never have any lines or spots whatsoever. 



Besides the complications introduced by variegation, we 

 have also to consider those introduced by retrogression. 

 Flowers which have reached a given stage in the pro- 

 gressive scale of colouration often show a tendency to fall 

 back to a lower stage. When this tendency is of the 

 nature of a mere temporary reversion (that is to say, when 

 it affects only a few individuals, or a casual variety), it 

 may conveniently be described as Relapse. When, how- 

 ever, it affects a whole species, and becomes fixed in the 

 species by a new and presumably lower adaptation, it 

 may best be styled Retrogression. 



Primary yellow (lowers, like the buttercups and poten- 

 tillas, show little or no tendency to vary in colour in a 

 state of nature. They have never passed through any 

 earlier stage to which they can revert ; and they are not 

 likely to strike out a new hue for themselves. 



Some white flowers, on the other hand, show a decided 

 tendency occasionally to revert to yellow, especially in the 

 simpler orders. Erysimum orientate varies from white to 

 pale primrose. Raphanus rapkanistrum, as already noted, 

 is usually even lilac, often white, and on the sea-shore 

 yellow. The white cistuses often revert to a pale sallow 

 tinge. In some roses, the throat becomes yellow in certain 

 specimens. Stitchwort occurs yellow near Exeter. In 



Fig. 21.— Flower of Indian Cres~, orange with dark lines : the honey-guides 

 point directly towards the long spur. 



several other cases, stray yellow specimens of normally 

 white species arc not uncommon. 



Pink and red flowers almost invariably revert in many 

 individuals to white. A few typical instances must suffice. 

 All the British ro;e^ are reddish pink or white. So are 

 Saponaria officinalis, and many pinks. Malva moschata 

 runs from rose-coloured to white ; .'/. rotundifolia from 

 pale lilac-pink to whitish. Erodium cicutarium has rosy 

 or white petals ; all the geraniums occasionally produce 

 very pale flowers. White varieties of heaths are frequent 

 in the wild state. Where the red or purple is very deeply 

 engrained, however, as in labiates, reversion to white 

 occurs less commonly. But almost all pink or red 

 flowers become white with the greatest readiness under 

 cultivation. 



Blue flowers in nearly every case produce abundant 

 red, pink, and white varieties in a state of nature. It 

 would seem, indeed, as though this highest development 

 of colour had not yet had time thoroughly to fix itself in 

 the constitution of most species. Hence individual re- 

 version is here almost universal as an occasional incident 

 in every species. The columbine (Aquilcgia vulgaris) is 

 blue or dull purple, sometimes red or white.' The larkspur 

 {Delphinium ajacis) often declines from blue to pink or 

 white. The monkshood (Aconitum napellus) is an ex- 

 tremely deep blue, very rarely white. White violets 

 everybody knows well. The rampions (Phytcuma) vary 

 from blue to white; so do many of the campanulas. 



Centiana campestris is sometimes white. In most Bora- 

 ginece — for example, in borage, viper's bugloss, and forget- 

 me-not — pink and white varieties are common. Pink and 

 white 1 eronicas also occur in abundance among normally 

 blue species. Prunella vulgaris occasionally produces 

 rosy or white blossoms. White wild hyacinths are often 

 gathered. Many other cases will suggest themselves to 

 every practical botanist. 



Blue flowers, however, very seldom revert to yellow. 

 As a rule, the blue goes back only as far as those shades 

 from which it has more recently been developed. This 



Fig. 



:. — Spotted Orchid, rurple w ith white patches: typ? of highly tie 

 loped bilateral monocotyledons. 



is, perhaps, the true rationale of De Candolle's law of 

 xanthic and cyanic types. 



With the light thus cast upon the question to guide us, 

 we may pass on to the general consideration of Retro- 

 gression in colours. Certain species uf advanced families 

 have apparently found it advantageous in certain circum- 

 stances to revert to colours lower in the scale than the 

 normal hue of their congeners. The reasons for such 

 Retrogression are often easy enough to understand. 



We may take the evening campion (Lychnis vespertina) 

 as a good example. This white flower, as we saw, is evi- 



Fig. 23.— Section of Deadneuk, relrogressionary white, with dark spots 

 1 n lip. Fig. ij.-Coinmon Toadflax, yellow, with the lip orange, acting 

 as guide to the hjney concealed in the long spur. 



dently descended from the red day campion (Lychnis 

 diurnia), because it is still often pale pink, especially 

 towards the centre, verging into white at the edge. But 

 it has found it convenient to attract moths and be ferti- 

 lised by them ; and so it has lost its pinkness, because 

 white is naturally the colour best seen by crepuscular in- 

 sects in the dusky light of evening. Sir John Lubbock 

 notes that such evening flowers never have any spots or 

 lines as honey-guides on the petals. 



The evening primrose (CEnothera biennis) is another 



