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Note on the Competition of Males (pp. 282-283).
(1) Miss BENSON of Addington Park, Croydon, has informed me that she had a mule canary-goldfinch, a very fine bird, much like a goldfinch but larger, and which sung very loudly and well. This bird frequently attracted wild goldfinches to it, which she thinks were hens, as they did not sing. On one occasion she put this bird into a cage with two other goldfinches, when he immediately drove away the male and monopolised the attentions of the female.
In this case we have a combination of larger size and more powerful song with somewhat finer colours, and the fact that female goldfinches were attracted towards him does not seem very remarkable. Whether the special attraction was the song, the large size, or the colour, there is no evidence to show.
(2) In the Narrative of the Cruise of the Challenger (vol. i. p. 890) is the following interesting statement: “Mr. Fell has watched the habits of the wild horses in Lafonia (Falkland Islands) closely. The strong and active horses each guard a herd of mares ; they keep the closest watch over them, and if one strays at all, drive her back into the herd by kicking her. The young horses live in herds apart, but the more vigorous ones are always on the look-out to pick up a mare from the herds of the older ones, and drive her off with them, and they sometimes gather a few mares and hold them for a short time till they are recaptured from them. When they think they are strong enough, they try the strength of the old horses in battle, and eventually each old horse is beaten by some rival and displaced ; the fighting is done mainly with the tusks, front to front, and not with the heels. Thus the most active and strongest males are naturally selected for the continuation of the herds."
In this case it is clear that there is very little opportunity for the mares to exercise any choice.
Note on the Exercise of Choice by Female Birds (p. 287). Mr. W. Storrs Fox of Bakewell, who has kept many birds in confinement and has also closely observed them in a state of nature,
writes me that he “has never come across any evidence in favour of choico exercised by females.” And he adds: “ All my own observations of birds in nature go to show that the choice of a mate rests wholly with the cock bird.”
· Note on Correlation of Colour with Health and Vigour (pp. 293-294).
The late Mr. J. Jenner Weir, who for many years kept large numbers of cage-birds and carefully watched their habits, has given me some valuable information bearing on the above question. He says: “ The four linnet-like birds, the linnet, twite, red pole, and mealy redpole, have red produced in the males in spring—in the first, third, and fourth on the head and breast, and in the second on the rump. Now none of the inales of these four species ever regain the red colour after moulting in captivity, which supports your view of the more vigorous males being the more richly coloured, for you cannot in captivity obtain the robust health which characterises the wild bird. In the chaffinch, bullfinch, and other allied birds, although the red is reproduced in captivity it is more or less of an orange tint. Again, I have in my garden four very healthy oyster-catchers : two I have had two years, but they never obtain the red legs of the wild bird, which is so distinctive a character that the genus derives its name, Hæmatopus, from this coloration.”
These facts certainly support the view that brilliancy of colour in male birds is closely connected with health and vigour, and until careful experiments are made we cannot tell whether it is this health and vigour, or the colour that accompanies it and which therefore becomes an indication of it, that is attractive to the females.
THE SPECIAL COLOURS OF PLANTS : THEIR ORIGIN
The general colour relations of plants—Colours of fruits—The meaning of
nuts—Edible or attractive fruits—The colours of flowers--Modes of securing cross-fertilisation—The interpretation of the facts—Summary of additional facts bearing on insect fertilisation-Fertilisation of flowers by birds-Self-fertilisation of flowers-Difficulties and contradictions—Intercrossing not necessarily advantageous-Supposed evil results of close interbreeding-How the struggle for existence acts among flowers-Flowers the product of insect agency-Concluding remarks on colour in nature.
THE colours of plants are both less definite and less complex than are those of animals, and their interpretation on the principle of utility is, on the whole, more direct and more easy. Yet here, too, we find that in our investigation of the uses of the various colours of fruits and flowers, we are introduced to some of the most obscure recesses of nature's workshop, and are confronted with problems of the deepest interest and of the utmost complexity.
So much has been written on this interesting subject since Mr. Darwin first called attention to it, and its main facts have become so generally known by means of lectures, articles, and popular books, that I shall give here a mere outline sketch, for the purpose of leading up to a discussion of some of the more fundamental problems which arise out of the facts, and which have hitherto received less attention than they deserve.
The General Colour Relations of Plants. The green colour of the foliage of leafy plants is due to the existence of a substance called chlorophyll, which is almost universally developed in the leaves under the action of light. It is subject to definite chemical changes during the processes of growth and of decay, and it is owing to these changes that we have the delicate tints of spring foliage, and the more varied, intense, and gorgeous hues of autumn. But these all belong to the class of intrinsic or normal colours, due to the chemical constitution of the organism; as colours they are unadaptive, and appear to have no more relation to the wellbeing of the plants themselves than have the colours of gems and minerals. We may also include in the same category those algæ and fungi which have bright colours—the “red snow” of the arctic regions, the red, green, or purple seaweeds, the brilliant scarlet, yellow, white, or black agarics, and other fungi. All these colours are probably the direct results of chemical composition or molecular structure, and, being thus normal products of the vegetable organism, need no special explanation from our present point of view; and the same remark will apply to the varied tints of the bark of trunks, branches, and twigs, which are often of various shades of brown and green, or even vivid reds or yellows.
There are, however, a few cases in which the need of protection, which we have found to be so important an agency in modifying the colours of animals, has also determined those of some of the smaller members of the vegetable kingdom. Dr. Burchell found a mesembryanthemum in South Africa like a curiously shaped pebble, closely resem. bling the stones among which it grew ;1 and Mr. J. P. Mansel Weale states that in the same country one of the Asclepiadeæ has tubers growing above ground among stones which they exactly resemble, and that, when not in leaf, they are for this reason quite invisible. It is clear that such resemblances must be highly useful to these plants, inhabiting an arid country abounding in herbivorous mammalia, which,
1 Burchell's Travels, vol. i. p. 10.
2 Nature, vol. iii. p. 507.
in times of drought or scarcity, will devour everything in the shape of a fleshy stem or tuber.
True mimicry is very rare in plants, though adaptation to like conditions often produces in foliage and habit a similarity that is deceiving. Euphorbias growing in deserts often closely resemble cacti. Seaside plants and high alpine plants of different orders are often much alike ; and innumerable resemblances of this kind are recorded in the names of plants, as Veronica epacridea (the veronica like an epacris), Limnanthemum nymphæoides (the limnanthemum like a nymphæa), the resembling species in each case belonging to totally distinct families. But in these cases, and in most others that have been observed, the essential features of true mimicry are absent, inasmuch as the one plant cannot be supposed to derive any benefit from its close resemblance to the other, and this is still more certain from the fact that the two species usually inhabit different localities. A few cases exist, however, in which there does seem to be the necessary accordance and utility. Mr. Mansel Weale mentions a labiate plant (Ajuga ophrydis), the only species of the genus Ajuga in South Africa, which is strikingly like an orchid ; while a balsam (Impatiens capensis), also a solitary species of the genus in that country, is equally like an orchid, growing in the same locality and visited by the same insects as other orchids. As both these genera of plants are specialised for insect fertilisation, and both of the plants in question are isolated species of their respective genera, we may suppose that, when they first reached South Africa they were neglected by the insects of the country; but, being both remotely like orchids in form of flower, those varieties that approached nearest to the familiar species of the country were visited by insects and cross-fertilised, and thus a closer resemblance would at length be brought about. Another case of close general resemblance, is that of our common white deadnettle (Lamium album) to the stinging-nettle (Urtica dioica); and Sir John Lubbock thinks that this is a case of true mimicry, the dead-nettle being benefited by being mistaken by grazing animals for the stinging-nettle.
· Flowers, Fruits, and Leaves, 1»128 (Fig. 79).