=Incipient Organs=.--Again, Mivart has drawn attention to another difficulty in the way of natural selection as an explanation even of useful organs. Darwin does not, of course, attempt to account for the _origin_ of varieties. As we have already seen, he assumes divergent variation of offspring as the necessary material on which natural selection operates. He who shall explain the origin of varieties will have made another great step in completing the theory of evolution. But not only does not natural selection explain the _origin_ of varieties, but neither can it explain the _first steps_ of advance toward usefulness. An organ must be already useful before natural selection can take hold of it to improve it. It can not make it useful, but only _more useful_. For example, if fins commenced as buds from the trunk, it is difficult to see how they could be of any use, and therefore how they could be improved by natural selection until they were of considerable size, and especially until muscles were developed to move them. Until that time they would seem to be a hindrance to be removed by natural selection, instead of a use to be preserved and improved. It would seem that many organs must have passed through this _incipient stage_, in which their use was prospective. Much that is very interesting might be said on these and similar points of difficulty, but all this lies entirely aside from the scope of this work. As already said, these are not objections to evolution or derivation, but only to _Darwinism_, or any other special theory, as a _sufficient explanation_ of the process of evolution. They only show that we do not yet fully understand this process; that there are still other and perhaps greater factors of evolution than is yet dreamed of in our philosophy. In the foregoing chapters on special evidences, and especially in the last two, the reader will observe many points of doubt, discussion, and difference of opinion. Let it not be concluded on that account that the _law of evolution_ is still in the region of uncertainty. It can not be too strongly insisted on that the fact of evolution as a universal law must be kept distinct from the causes, the factors, the conditions, the processes, of evolution. The former is certain, the latter are still imperfectly understood. PART III. _THE RELATION OF EVOLUTION TO RELIGIOUS THOUGHT._ CHAPTER I. INTRODUCTORY.