But if, admitting this, we proceed to consider how the special aptitude of vespertina is constituted, or what it is that puts diurna at a disadvantage, we find ourselves quite unable to show the slightest connexion between the success of one or the failure of the other on the one hand, and the specific characteristics which distinguish the two forms on the other. The orthodox Selectionist would, as usual, appeal to ignorance. We ask what can vespertina gain by its white flowers, its more lanceolate leaves, its grey seeds, its almost erect capsule-teeth, its longer fruits, which diurna loses by reason of its red flowers, more ovate leaves, dark seeds, capsule-teeth rolled back, and shorter fruits? We are told that each of these things may affect the viability of their possessors. We cannot assert that this is untrue, but we should like to have evidence that it is true. The same problem confronts us in thousands upon thousands of examples, and as time goes on we begin to feel that speculative appeals to ignorance, though dialectically admissible, provide an insufficient basis for a proposition which, if granted, is to become the foundation of a vast scheme of positive construction.