=Objection answered.=--This view completes the answer to an objection which is often made to evolution: “If natural species are formed by transmutation, why is it we do not find intermediate links? Why is not organic nature made up only of individual forms, shading insensibly into each other in such wise that classification becomes a mere device to handle more conveniently complex material? Why is it that groups, especially species, are marked out with hard and fast lines?” We have heretofore answered this by saying that intermediate forms are eliminated. So they are, but how? Dr. Romanes’s idea of physiological selection largely answers this. It is by the funding of _ordinary_ varieties into a common parental stock by crossing, and separating _specific_ varieties by cross-sterility. Thus the organic field is broken up into points about which variations oscillate. As every mass of matter, when closely examined, is found to consist of aggregations about centers of _cohesive_ attraction as discrete granules or crystals, and only exceptionally do we find a homogeneous vitreous structure; even so organic forms aggregate about points of _sexual_ attraction, and the whole mass consists of discrete species, and only exceptionally--i. e., in domestication--do we find insensible shadings. Now, species are the smallest aggregate of individuals, as granules are of molecules. Species are more distinctly marked out by hard and fast lines than are other taxonomic groups only because they are the _last_, going downward, that are cross-sterile--because right here is the change from cross-sterility to cross-fertility. If this view be true, then in _the same locality_ species ought to be always distinct and without shadings. If we find shadings at all, it ought to be in intermediate geographical regions, where isolation is not sexual but physical. Now, this is exactly what we find to be the fact. _Innumerable examples of such intermediate forms in intermediate geographical regions_ are now known, especially among birds and reptiles, and examples have so increased in modern times, by closer study, that naturalists, especially ornithologists, have been compelled to resort to a trinomial nomenclature in order to designate these geographical sub-species.[33] If any further explanation is necessary, it will probably be found in the following suggestions: