But the philosophical significance of natural selection lies in the fact, that it shows us how to explain the origin of useful, well-adapted structures purely by mechanical forces and without having to fall back on a _directive_ force. We are thus for the first time in a position to understand, in some degree, the marvellous adaptation of the organism to an end, without having to call to our aid any supernaturally intrusive force on the part of the Creator. We understand now how, in a purely mechanical way, through the forces always at work in nature, all forms of life must conform to, and adapt themselves precisely to the conditions of their life, since only the best possible is preserved, and everything less good is continually being rejected. Before I go on to expound in detail the phenomena which we refer to natural selection, I must briefly state that Darwin did not ascribe to natural selection by any means all the changes which have taken place in organisms in the course of time. On the one hand, he ascribed a not inconsiderable importance to the correlated variations we have already mentioned; still more, however, he relied on the direct influence of altered conditions of life, whether these consist in climatic and other changes in the environment, or in the assumption of new habits, and the increased or diminished use of individual parts and organs thereby induced. He recognized the principle so strongly emphasized by Lamarck, of use and disuse as a cause of heritable increase or decrease of the exercised or neglected part, though he did so with a certain reserve. I shall return later to these factors of modification, and shall then attempt to show that these too are to be referred to processes of selection, which are, however, of a different order from the phenomena which the Darwin-Wallace principle of natural selection serves to interpret. But, in the first instance, it appears to me to be necessary to show how far the Darwin-Wallace interpretation will suffice, and in the next lectures we shall occupy ourselves with this question exclusively. LECTURE IV THE COLORATION OF ANIMALS AND ITS RELATION TO THE PROCESSES OF SELECTION Biological significance of colours--Protective colours of eggs--Animals of the snow-region--Animals of the desert--Transparent animals--Green animals--Nocturnal animals--Double colour-adaptation--Protective marking of caterpillars--Warning markings--Dimorphism of colouring in caterpillars--Shunting back of colouring in ontogeny--'Sympathetic' colouring in diurnal Lepidoptera--In nocturnal Lepidoptera--Theoretical considerations--The influence of illumination in the production of protective colouring, _Tropidoderus_--Harmony of protective colouring in minute details--_Notodonta_--Objections--Imitation of Strange objects, _Xylina_--Leaf-butterflies, _Kallima_--_Hebomoja_--Nocturnal Lepidoptera with leaf-markings--Orthoptera resembling leaves--Caterpillars of the Geometridæ.