[25] See [Chapter VII.] of this volume.
Again, there is reason to believe that some portion of the large size and corresponding showiness of tropical flowers is due to their being fertilized by very large insects and even by birds. Tropical sphinx-moths often have their probosces nine or ten inches long, and we find flowers whose tubes or spurs reach about the same length; while the giant bees, and the numerous flower-sucking birds, aid in the fertilization of flowers whose corollas or stamens are proportionately large.
Recent Views as to Direct Action of Light on the Colours of Flowers and Fruits.—The theory that the brilliant colours of flowers and fruits is due to the direct action of light, has been supported by a recent writer by examples taken from the arctic instead of from the tropical flora. In the arctic regions vegetation is excessively rapid during the short summer, and this is held to be due to the continuous action of light throughout the long summer days. “The further we advance towards the north the more the leaves of plants increase in size as if to absorb a greater proportion of the solar rays. M. Grisebach says, that during a journey in Norway he observed that the majority of deciduous trees had already, at the 60th degree of latitude, larger leaves than in Germany, while M. Ch. Martins has made a similar observation as regards the leguminous plants cultivated in Lapland.”[26] The same writer goes on to say that all the seeds of cultivated plants acquire a deeper colour the further north they are grown, white haricots becoming brown or black, and white wheat becoming brown, while the green colour of all vegetation becomes more intense. The flowers also are similarly changed: those which are white or yellow in central Europe becoming red or orange in Norway. This is what occurs in the Alpine flora, and the cause is said to be the same in both—the greater intensity of the sunlight. In the one the light is more persistent, in the other more intense because it traverses a less thickness of atmosphere.
[26] Revue des Deux Mondes, 1877. “La Vegetation dans les hautes Latitudes,” par M. Tisserand.
Admitting the facts as above stated to be in themselves correct, they do not by any means establish the theory founded on them; and it is curious that Grisebach, who has been quoted by this writer for the fact of the increased size of the foliage, gives a totally different explanation of the more vivid colours of Arctic flowers. He says—“We see flowers become larger and more richly coloured in proportion as, by the increasing length of winter, insects become rarer, and their co-operation in the act of fecundation is exposed to more uncertain chances.” (Vegetation du Globe, vol. i. p. 61—French translation.) This is the theory here adopted to explain the colours of Alpine plants, and we believe there are many facts that will show it to be the preferable one. The statement that the white and yellow flowers of temperate Europe become red or golden in the Arctic regions must we think be incorrect. By roughly tabulating the colours of the plants given by Sir Joseph Hooker[27] as permanently Arctic, we find among fifty species with more or less conspicuous flowers, twenty-five white, twelve yellow, eight purple or blue, three lilac, and two red or pink; showing a very similar proportion of white and yellow flowers to what obtains further south.
[27] “On the Distribution of Arctic Plants,” Linn. Trans. vol. xxiii. (1862.)
We have, however, a remarkable flora in the Southern Hemisphere which affords a crucial test of the theory of greater intensity of light being the direct cause of brilliantly-coloured flowers. The Auckland and Campbell’s Islands south of New Zealand, are in the same latitude as the middle and the south of England, and the summer days are therefore no longer than with us. The climate though cold is very uniform, and the weather “very rainy and stormy.” It is evident, then, that there can be no excess of sunshine above what we possess; yet in a very limited flora there are a number of flowers which—Sir Joseph Hooker states—are equal in brilliancy to the Arctic flora. These consist of brilliant gentians, handsome veronicas, large and magnificent Compositæ with purple flowers, bright ranunculi, showy Umbelliferæ, and the golden flowered Chrysobactron Rossii, one of the finest of the Asphodeleæ.[28] All these fine plants, it must be remembered, are peculiar to these islands, and have therefore been developed under the climatal conditions that prevail there; and as we have no reason to suppose that these conditions have undergone any recent change we may be quite sure that an excess of light has had nothing to do with the development of these exceptionally bright and handsome flowers. Unfortunately we have no information as to the insects of these islands, but from their scarcity in New Zealand we can hardly expect them to be otherwise than very scarce. There are however two species of honey-sucking birds (Prosthemadera and Anthornis) as well as a small warbler (Myiomoira), and we may be pretty sure that the former at least visit these large and handsome flowers, and so effect their fertilization. The most abundant tree on the islands is a species of Metrosideros, and we know that trees of this genus are common in the Pacific islands, where they are almost certainly fertilized by the same family of Meliphagidæ or honey-sucking birds.
[28] Coloured figures of all these plants are given in the Flora Antarctica, vol. i.
I have now concluded this sketch of the general phenomena of colour in the organic world. I have shown reasons for believing that its presence, in some of its infinitely-varied hues, is more probable than its absence; and that variation of colour is an almost necessary concomitant of variation of structure, of development, and of growth. It has also been shown how colour has been appropriated and modified both in the animal and vegetable worlds for the advantage of the species in a great variety of ways, and that there is no need to call in the aid of any other laws than those of organic development and “natural selection” to explain its countless modifications. From the point of view here taken it seems at once improbable and unnecessary that the lower animals should have the same delicate appreciation of the infinite variety and beauty—of the delicate contrasts and subtle harmonies of colour, which are possessed by the more intellectual races of mankind, since even the lower human races do not possess it. All that seems required in the case of animals, is a perception of distinctness or contrast of colours; and the dislike of so many creatures to scarlet may perhaps be due to the rarity of that colour in nature, and to the glaring contrast it offers to the sober greens and browns which form the general clothing of the earth’s surface, though it may also have a direct irritating effect on the retina.
The general view of the subject now given must convince us that, so far from colour being—as it has sometimes been thought to be—unimportant, it is intimately connected with the very existence of a large proportion of the species of the animal and vegetable worlds. The gay colours of the butterfly and of the alpine flower which it unconsciously fertilizes while seeking for its secreted honey, are each beneficial to its possessor, and have been shown to be dependent on the same class of general laws as those which have determined the form, the structure, and the habits of every living thing. The complex laws and unexpected relations which we have seen to be involved in the production of the special colours of flower, bird, and insect, must give them an additional interest for every thoughtful mind; while the knowledge that, in all probability, each style of coloration, and sometimes the smallest details, have a meaning and a use, must add a new charm to the study of nature.