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such as nutrition, cold, &c., and these may affect the young, or even the embryo, or the egg before it is laid by the parent. No two animals can ever come into existence under absolutely identical conditions; neither can any two after birth be exposed to absolutely the same conditoins.

Variation under domestication is the rule instead of the exception, and occurs more or less in every direction. Consider, for instance, the extraordinary variations in size and mode of growth of the cabbage; the solid heads of foliage utterly unlike any plant in a state of Nature; the curiously wrinkled leaves of the savoy, the purple leaves of the pickling cabbage, the compact heads of flowers of the broccoli and cauliflower, the curious stem of the Kohlrabi, which grows like a turnip. Again, of the apple there are at least a thousand varieties known, all descended from the common crab-apple. In fact, as Wallace says, "there is hardly an organ or a quality in plants or animals which has not been observed to vary; and further, whenever any of these variations have been useful to man, he has been able to increase them to a marvellous extent by the simple process of always preserving the best varieties to breed from."

Limits to variation must of course exist, and it is evident that up to some point or other variations must be predetermined on definite lines. The inconstancy of chemical composition or instability is specially characteristic of living things. Variations are spoken of as accidental, not in the sense of their not being all due to natural causes, but inasmuch as they are accidental in relation to the sifting process of natural selection.

E. NATURAL SELECTION, OR THE SURVIVAL OF THE FITTEST. Those animals which are most in harmony with their surroundings will survive. Just as in the breeding of animals by artificial selection, those animals are selected to survive which have certain favourable peculiarities, usually too slight for any but a practised eye to detect; so under natural conditions the possession of some useful variation, such as a slight increase of speed, or power of endurance or strength, or a keener sense of vision, will determine which shall be the survivors in a large herd of animals.

The action of natural selection is well shown by the following example. Many insects of Madeira have either lost their wings, or had them so much reduced as to be useless for flight, while their allies in Europe have them well developed. The explanation of this is that Madeira, like other temperate oceanic islands, is much exposed to sudden gales, and the most fertile land being near the coast, the insects if able to fly are liable to be blown out to sea and lost. Year after year the individuals which had the shortest wings, or which used them least, would have an advantage, and so would survive. Hence the survival in the island of the insects with the smallest wings.

In Kerguelen Island, one of the stormiest places on the globe and a place entirely without shelter, all the insects are incapable of flight, and most of them are entirely destitute of wings. These insects include a moth, several flies, and many beetles. Now these are the descendants of winged insects, which must have reached the island by flying, and gradually lost the power of flight, as in the insects of Madeira.

The importance of small variations. We are apt to overlook the importance that slight variations may have, which is well shown in the artificial breeding of animals. So it is with human affairs, where important points, such as the fate of a Ministry, or even the determination of peace or war between two countries, often depends on side issues. In trade, accidental variations may determine success by attracting attention. The success of a novel, play or oratorio is often impossible to predict, and often depends on a mere caprice. Change for the mere sake of change may involve the misery or even death of thousands, and cause alternating periods of great prosperity and greater distress. This is well seen in the changes of fashion in dress, which in the case of the feathers for ladies' hats, or a particular kind of fur, may mean destruction and wholesale slaughter, even to extermination, of particular animals.

INHERITANCE. - The more favoured ones will not only survive, but will tend to hand down to their descendants their special advantages; and of these descendants some will have these special peculiarities in a less marked degree than their parents, some equally and others more strongly marked. The latter will in the long run survive, if the further development of this special advantage confers further benefit on the individual. The whole theory of the breeding of animals depends on inheritance. For instance, the pedigrees of race-horses are kept with most scrupulous care, and enormous prices are paid for horses for breeding purposes. So it is with pigs, poultry, dogs, and cattle, and in the improvements effected in fruits and flowers. Not only are good characters inherited, but bad ones also, and even diseases and malformations, such as insanity, gout, short-sight, cataract, and colour-blindness, among

men.

F. THE INFLUENCE OF ENVIRONMENT. --- We have seen that the struggle for existence results in the survival of the fittest. Now, if the conditions remain permanent there is no reason to suppose that the race would alter, for the fittest now would be so a thousand years hence, provided the external conditions did not change in the meantime. Variations would no doubt occur, but as none of these would confer an advantage, they would not be preserved. In a very few cases this is so, but constant change is the rule. For instance, consider the change effected in Australia by the arrival of civilised man with his dogs, horses, &c., resulting in the aboriginal inhabitants, human and animal alike, being killed off by competition. Man's influence is no doubt great; but other influences are still more potent.

Here we derive much assistance from the evidence afforded by geology, which tells us that, as regards the earth we live in, things were not always as we find and know them now. The marks on boulders and deposits of glacial mud and clay, show that these boulders have been brought from afar, that their only possible means of transit was by glaciers, and

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THE DARWINIAN THEORY

hence that our climate was once much colder than it is at the present time. If, on the other hand, we turn to Greenland, which is now in the glacial condition, we find beneath the ice, beds containing fossil plants, showing the former existence in Greenland of such plants as the chestnut, oak, plane, beech, and poplar; nay even the magnolia, vine, walnut, and plum; proving the former existence not only of a moderate, but of a warm climate.

Geology shows us that the boundaries of land and sea are not constant; for instance, that Britain and France were once united, and that the sea is encroaching on the land on one side, while the land is encroaching on the sea on the other. The crust of the earth is made up chiefly of rocks deposited under water; therefore where there is now dry land, there must once have been open sea. Geology further shows that these changes have not been of a sudden cataclysmal character, but gradual ones, changes which are actually in progress at the present day, and which must always have been going on since the earth began.

The last link in the chain is now complete. Owing to incessant geological change in environment, variations in structure, previously useless or harmful, become advantageous, and their possessors thereby triumph and survive, and hand down their advantages to their descendants; thereby in course of time causing structural modifications of greater or less extent in the race. All Nature is in a condition of more or less unstable equilibrium. The action of environment is indirect, and changed conditions of

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