IN order to understand and interpret correctly the operation of natural selection in producing new species and maintaining them, by "the preservation of favoured races in the struggle for life" (to use Darwin's words), we must take a wide and, at the same time, a minutely accurate survey of the living world. We must seek out the evidences of this operation and use the imagination in forming conceptions as to the varied steps of the process and the results which are likely to ensue from it at different stages and in different conditions. We cannot interpret the existing structures and behaviour of living things by the use of a simple formula, such as that set up by some writers who have not properly studied Mr. Darwin's works, and declare that, according to him, all structures and behaviours which we observe in living things are perfect and the finished result of survival of the ideally fittest variations. Plants and animals are so complex (as no one has shown more clearly than Darwin), not only in their structure but in the chemical and physical action and interaction of their living parts, that in the course of the ages during which the present species have been, step by step, fashioned in the endless vicissitudes of a changing world, many of them have retained structures or chemical constitutions which once were useful but now are useless, or even positively injurious. Even injurious structures or behaviours may be retained and inherited by a species of plant or animal, if, on the whole, the other accompanying modifications of structure are valuable--that is, of "life-saving" value, so that, "on the whole," the race is favoured by selection in the struggle for existence. In species which have but lately acquired dominance or are brought by their success into novel conditions, we may, and do, find old structures and behaviours still persisting which are injurious, not yet, as it were, "cleaned up" and got rid of as they would be in the course of further long periods of selection. Such species become established, and may even acquire a definite stability, because the injurious structures or behaviours which they have retained are of little or no account as compared with the other advantageous characters which the species have developed. The term "disharmonies" is applied to such injurious characters, consisting in a certain want of harmony (in minor respects) between the structure of an organism and the conditions in which, nevertheless, it thrives.