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Among birds, the Cuckoo is undoubtedly protected by its similarity to a hawk in appearance and in mode of flight. In the snakes, again, the harmless ones are said to mimic the venomous.

The conditions necessary in order to effect mimicry are given by Wallace as follows :

(1) The two species, the imitating and the imitated, must occur in the same locality.

(2) The imitating species must be the more

defenceless.

(3) The imitating species must be less numerous than the imitated, in individuals.

(4) The imitating species must differ from the bulk of its allies.

(5) The imitation, however minute, is external only, never extending to internal characters or to such as do not affect external appearance.

The mode of acquisition of mimicry is by the gradual action of natural selection, and must have been accidental in the first instance.

RECOGNITION MARKINGS.

These are closely allied to warning colours, and their purpose is to facilitate recognition, not by enemies, but by friends. They are seen especially in gregarious animals, and specific markings and colours are very probably in many cases not protective, but for recognition. A good instance of this class of colouring is seen in the upturned white tail of the Rabbit, which, although making it conspicuous to its enemies as well as friends, is probably a signal of danger to the other rabbits; and when feeding together, in accordance with their social habits, soon after sunset or on moonlight nights, the upturned tails of those in front serve as guides to those behind to run home on the appearance of an enemy. Many birds, antelopes, and other animals, have markings believed to serve a similar purpose, and probably the principle of distinctive colouring for recognition has something to do with the great diversity of colour met with in butterflies.

EPIGAMIC COLORATION.

This is seen in mature animals, especially in butterflies and birds, where the two sexes differ markedly as regards colour. As a general rule, the male is of the same hue as the female, but of a deeper and more intensified colour; for instance, in thrushes, hawks, and in the Emperor moth. Sometimes patches of colour found in the males are absent in the females, as in the Orange-tip butterfly. In some cases there are more extreme differences, as in the drake, peacock, cocks and hens, pheasants and Bird of Paradise; gay colours being the special privilege of the male. (Fig. 26.)

It is curious to note how with man the conditions are reversed, for the female butterfly or bird is as a rule larger and plainer than her mate. So it is with the organs of voice; the male cricket or grasshopper can alone produce sound, and many female birds have no song. The power of talking was originally the exclusive possession of the males-a

privilege that with us they have long had to surrender.

The origin of the bright colours of the male may possibly be due to the selective action of the sexes

FIG. 26.

Bird of Paradise, male and female.

on one another. As among deer now, and men in several countries, the best fighters carry off the prizes; so among birds and butterflies the most gaily or tastefully arrayed males are the most highly favoured, and have the best chance of securing the most eligible mates. In the case of the Argus pheasant, the wing feathers are enormously elongated and of marvellously beautiful colouring; while during courtship the male erects his tail like a fan, displaying his glories to their best advantage. Similar examples are afforded by humming-birds.

There is no possible doubt as to the appreciation of colour by animals, and the chief differences between men and butterflies would appear to lie in the infinitely better taste displayed by the latter in the selection and combination of colour, both as regards marvellously delicate gradations of tint, and as regards daring combinations of strongly-contrasted colours. It is in fact as rare for a bird or butterfly to offend against good taste in matters of colour, as it is for a man to conform to it.

The theory of sexual selection, which was proposed by Darwin as supplementary to natural selection, is disputed by Wallace, who holds that brighter colour is the physical equivalent of greater vigour.

THE COLOURS OF FLOWERS AND FRUIT.

The essential parts of a flower are the ovary, in which the ovules are produced, and the anthers, in which the pollen is contained; and in order that the ovule may give rise to a seed-i.e., to something capable of growing into a new plant-it must be fertilised by the pollen. There is a great advantage as regards the number of seeds produced, and the vigour of the offspring, if the ovules are fertilised by

pollen, not from the same flower, but from a different flower or plant. This cross fertilisation is always highly beneficial and often absolutely essential. It is effected mainly by the agency of insects, especially bees, flies, and butterflies. These are induced to visit the flowers by bribes of honey secreted by the flower in such a position that, in order to reach it, the insect must brush against the anthers and get dusted with the pollen, by which, on visiting a second flower, fertilisation is effected.

The purpose of the coloured part of the flower is to form a conspicuous advertisement to insects of places where honey is to be found; and more detailed markings in the flower direct the insect towards the store of honey. Curiously ingenious contrivances are found in order to prevent selffertilisation, and to ensure that the insect shall effect its work properly.

A familiar instance is that of Orchis mascula, the spotted orchid, which is abundant in meadows and in damp places in open woods. This consists of a spike of flowers, the calyx of which is formed by three coloured sepals, and the corolla by three petals. One of the petals, called the labellum, is larger than the others, and forms a sort of landingstage. This is prolonged backwards into a spur-like nectary with spongy walls. The male organs consist of one anther with two cells, each of which contains a pollen mass. The ovary has three pistils, united together and twisted, ending above in two almost confluent stigmas. The third stigma forms the rostellum, a rounded projection overhanging the

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