the race into harmony with the conditions. In many cases, however, it is probable that this cannot be done; the internal organs may not vary quick enough, and then the animal will decrease in numbers, and finally become extinct. If a larger or more powerful beast is to be captured and devoured-as when a carnivore, which has hitherto preyed on antelopes, is obliged from their decreasing numbers to attack buffaloes—it is only the strongest who can struggle with and overcome such an animal-those with the most powerful claws and formidable canine teeth. Consequently, any variation in these respects will be selected; that is, it will give its possessor a better chance of life than his fellows of the same species, and among his offspring will probably be found some who inherit his peculiarities, perhaps in a more marked degree.

Let us suppose that a certain animal is exactly fitted to secure its prey, to escape from its enemies, to resist the inclemency of the seasons, and to rear a numerous and healthy offspring, all goes well so long as the outward conditions remain unchanged. But a series of cold winters comes on, making food scarce, and bringing an immigration of some other animals to compete with the former inhabitants of the district. The new immigrant is swift of foot, and surpasses its rivals in the pursuit of game, the winter nights are colder and require a thicker fur as a protection, and more nourishing food to keep up the heat of the system. Our supposed perfect animal is no longer in harmony with its environment, it is in danger of dying of cold or starvation; but among its offspring there may be some swifter than others, who will still manage to catch food enough; some hardier and more thickly furred, who can manage

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to keep warm enough: while the slow, the weak, and the thinly clad will soon die off. Again and again, in each succeeding generation, the same thing takes place; and by this natural process, which is so inevitable that it cannot be conceived not to act, those best adapted to live do live, and those least adapted die.

These are comparatively simple instances, but very often the interaction of climate, food supply, animal organization, conditions of plant life, &c., is intricately interwoven, and plants and animals most remote in the scale of nature are bound together by a web of complex relations. It would hardly be thought credible that the presence of a feline animal in large numbers in a district should determine the frequency of certain flowers in that district; yet this appears to be the case. The heartsease and red clover are dependent for their fertilization, and therefore their perpetuation, on the visits of humble bees; for other bees do not visit these flowers, not being able to reach the nectar. Hence we may infer, that if the whole genus of humble bees became extinct, or very rare in England, the heartsease and red clover would become very rare, if they did not wholly disappear. The number of humble bees in any district depends in a great degree on the number of field mice, which destroy their combs and nests, so much so that Mr H. Newman, who has long attended to the habits of humble bees, believes that more than two-thirds of them are thus destroyed all over England. The number of mice, again, is largely dependent, as every one knows, on the number of cats; and Mr Newman says, "Near villages and small towns, I have found the nests of humble bees more numerous than elsewhere, which I attribute to the number of cats which destroy the mice." So

then, the more cats the fewer mice; the fewer mice the more bees, the more bees the more heartsease and red clover, that is, the more cats the more of these flowers! And of course in all these struggles the quickest cats and quickest mice will have the advantage, and the bees with the longest probosces, and the flowers whose nectar is most easily reached; and therefore variations in these directions will be selected and perpetuated.

Protective Resemblances.-When Jehoshaphat accompanied Ahab in the war against Ramoth Gilead, the King of Judah put on his robes, and in consequence was singled out by the Syrian captains, and nearly lost his life; but Ahab disguised himself and went into the battle, and although an arrow struck him, it was a chance shot not aimed at him especially. It is often urged that the red coat of the British soldier renders him too conspicuous an object to the enemy, and that to escape unnecessary danger the clothing should be green or grey, assimilated to the colour of the ground he walks on, or the foliage among which he moves. The disguise which would save the soldier from the observation of his enemy would serve him also in another way, for it would quite as much enable him as an assailant to steal upon the foe unperceived. Nearly all birds and insects have enemies who are similarly capable of being deceived; the hawk, for instance, is so much guided by eyesight in taking its prey, that on parts of the continent persons are warned not to keep white pigeons, because they are the most liable to be seen and destroyed. When, therefore, we see leaf-eating insects green, and bark-feeders mottled grey, the alpine ptarmigan white in winter, the red grouse the colour of the heather, and the black grouse that of peaty earth, we must believe

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that these tints are of service to these birds and insects in preserving them from danger. Another step in advance, and we have insects which are formed as well as coloured so as exactly to resemble particular leaves, or sticks, or mossy twigs, or flowers; and in these cases very peculiar habits and instincts come into play to aid in the deception, and render the concealment more complete. Probably the reader has seen in some museum a specimen of the "walking-leaf," in which not only are the wings perfect imitations of leaves in every detail, but the thorax and legs are flat, dilated, and leaf-like, so that when the living insect is resting among the foliage on which it feeds, the closest observation is often unable to distinguish between the animal and the vegetable. The whole family of the Phasmida or Spectres, to which this insect belongs, is more or less imitative, and a great number of the species are called "walking-stick insects," from their singular resemblance to twigs and branches. Some of these are a foot long and as thick as one's finger, and their whole colouring, form, rugosity, and the arrangement of the head, legs, and antennæ, are such as to render them absolutely identical in appearance with dead sticks. They hang loosely about the shrubs in the forest, and have the extraordinary habit of stretching out their legs unsymmetrically, so as to render the deception more complete. One of these creatures, obtained by Mr Wallace in Borneo, was covered with foliaceous excrescences of a clear olive green colour, so as exactly to resemble a stick grown. over by a creeping moss or jungermannia. The Dyak who brought it to him assured him it was thus grown over, although alive, and it was only after minute examination that he could convince himself it was not so.

There can be no doubt that many birds have their colours assimilated to those of surrounding objects, so that they may escape the notice of their enemies, or may approach their prey unobserved. Mr Wallace remarks that it is only in the tropics, among forests which never lose their foliage, that we find whole groups of birds whose chief colour is green. In regard to birds which live on the ground, how difficult it is to see a partridge, snipe, woodcock, or certain plovers, larks, and night-jars, when crouched on the soil. Animals inhabiting deserts offer striking instances of this imitation, for the bare surface affords no concealment, and all the smaller quadrupeds, reptiles, and birds, depend for safety on their colours. Mr Tristram has remarked, in regard to the inhabitants of the Sahara, that all are protected by their isabelline or sand colour.

Time would fail us to go through the whole list of creatures "protected"— birds, snakes, toads, beetles, butterflies, caterpillars, &c., &c. No one can behold the upper surface of a flounder and overlook its resemblance to the sandy bed of the sea on which it lives. There are now (1870) in the aquarium of the Zoological Society some slender green pipe-fish, which fasten themselves to any object at the bottom by their prehensile tails, and float about with the current, looking exactly like some simple forms of sea-weed. In the same Zoological Gardens, how difficult it is sometimes to catch sight of the little green tree-frogs sitting on the leaves of a small plant enclosed in a glass case; yet how much better concealed must they be among the fresh green damp foliage of a marshy forest! The most arboreal lizards, the iguanas, are as green as the leaves. they feed upon, and the slender whip-snakes are rendered