We may say that the process of selection follows as a logical necessity from the fulfilment of the three preliminary postulates of the theory: variability, heredity, and the struggle for existence, with its enormous ratio of elimination in all species. To this we must add a fourth factor, the _intensification_ of variations which Darwin established as a fact, and which we are now able to account for theoretically on the basis of germinal selection. It may be objected that there is considerable uncertainty about this _logical_ proof, because of our inability to demonstrate the selection-value of the initial stages and the individual stages of increase. We have therefore to fall back on _presumptive evidence_. This is to be found in _the interpretative value of the theory_. Let us consider this point in greater detail. In the first place it is necessary to emphasize what is often overlooked, namely, that the theory not only explains the _transformations_ of species, it also explains _their remaining the same_; in addition to the principle of varying, it contains within itself that of _persisting_. It is part of the essence of selection, that it not only causes a part to _vary_ till it has reached its highest pitch of adaptation, but that it _maintains it at this pitch. This conserving influence of natural selection_ is of great importance, and was early recognised by Darwin; it follows naturally from the principle of the survival of the fittest. We understand from this how it is that a species which has become fully adapted to certain conditions of life ceases to vary, but remains "constant," as long as the conditions of life _for_ it remain unchanged, whether this be for thousands of years, or for whole geological epochs. But the most convincing proof of the power of the principle of selection lies in the innumerable multitude of phenomena which cannot be explained in any other way. To this category belong all structures which are only _passively_ of advantage to the organism, because none of these can have arisen by the alleged _Lamarckian principle_. These have been so often discussed that we need do no more than indicate them here. Until quite recently the sympathetic coloration of animals--for instance, the whiteness of Arctic animals--was referred, at least in part, to the _direct_ influence of external factors, but the facts can best be explained by referring them to the processes of selection, for then it is unnecessary to make the gratuitous assumption that many species are sensitive to the stimulus of cold and that others are not. The great majority of Arctic land-animals, mammals and birds, are white, and this proves that they were all able to present the variation which was most useful for them. The sable is brown, but it lives in trees, where the brown colouring protects and conceals it more effectively. The musk-sheep (_Ovibos moschatus_) is also brown, and contrasts sharply