merely chemical, the oxygen accumulated by night being used up by day.
But the most quick-witted insects would notice them, and we find that some plants have adopted them as dialects of the universal "flower-language." Fritz Müller describes a Brazilian lantana whose flowers last three days, and are yellow the first, red the second, purple the third. A few butterflies stick their tongues into the yellow and red, others visit only the red, none the purple. If the flowers fell the first day, the inflorescences would be less conspicuous; if the butterflies did not appreciate the change, they would lose time in searching for honey from old, functionless nectaries. What may be a purely chemical phenomenon in the hibiscus has become a constant and useful character. (Even more remarkable is the case described by Hildebrand of Eremurus, whose flowers open before the reproductive organs are mature. After the corolla is withered, stamens, stigma, and nectaries become fully developed, so the less intelligent insects, decoyed by the bright young flowers and finding there no honey, leave the inflorescence to the friends who love it because "it has opened its heart" to them.)
Tannin, which causes the disagreeable taste of many petals, is peculiarly abundant in cells which exhibit irritability—i. e., it is easily affected by outside agencies. Petals are delicate organs, and whatever irritable substance is in them is therefore the more easily stimulated. Add this to the fact of their youth, and it is natural that flowers should be pre-eminently variable, and altogether possible that certain stages may be seized upon and made hereditary by the selection of the innumerable army of hungry insects.
So, again, there is law all the way through. The operation of entirely natural and conceivable causes leads to the permanent establishment and combination of colors, odors, and forms which may be protective, repulsive, imitative, attractive, or unite several of these functions. And in the "continuous adjustment of internal to external conditions" is the evolution from alga to rose, which shall by no means stop with the rose. The song of the flowers is clear and true:
A power, more strong in beauty, born of us
And fated to excel us, as we pass
In glory that old darkness;
—For, 'tis the eternal law,