forms, and also of absolute degradation or degeneration. Serpents, for example, have been developed from some lizard-like type which has lost its limbs; and though this loss has enabled them to occupy fresh places in nature and to increase and flourish to a marvellous extent, yet it must be considered to be a retrogression rather than an advance in organisation. The same remark will apply to the whale tribe among mammals; to the blind amphibia and insects of the great caverns; and among plants to the numerous cases in which flowers, once specially adapted to be fertilised by insects, have lost their gay corollas and their special adaptations, and have become degraded into wind-fertilised forms. Such are our plantains, our meadow burnet, and even, as some botanists maintain, our rushes, sedges, and grasses. The causes which have led to this degeneration will be discussed in a future chapter; but the facts are undisputed, and they show us that although variation and the struggle for existence may lead, on the whole, to a continued advance of organisation; yet they also lead in many cases to a retrogression, when such retrogression may aid in the preservation of any form under new conditions. They also lead to the persistence, with slight modifications, of numerous lowly organised forms which are suited to places which higher forms could not fully occupy, or to conditions under which they could not exist. Such are the ocean depths, the soil of the earth, the mud of rivers, deep caverns, subterranean waters, etc.; and it is in such places as these, as well as in some oceanic islands which competing higher forms have not been able to reach, that we find many curious relics of an earlier world, which, in the free air and sunlight and in the great continents, have long since been driven out or exterminated by higher types.
Summary of the first Five Chapters.
We have now passed in review, in more or less detail, the main facts on which the theory of "the origin of species by means of natural selection" is founded. In future chapters we shall have to deal mainly with the application of the theory to explain the varied and complex phenomena presented by the organic world; and, also, to discuss some of the theories put forth by modern writers, either as being more fundamental than
