2. Can you doubt, after duly considering the circumstances under which allied species of plants have been differentiated--viz. in ninety-five per cent. of cases intimately commingled on common areas, and therefore under identical environments--that cross-infertility _must_ have arisen before or during the specific differentiation? 3. Can you doubt, after duly considering the facts of prepotency on the one hand and those of Jordan's physiological varieties on the other, that cross-infertility _does_ arise before or during the specific differentiation? 4. If you cannot express a doubt upon any of these points, can you explain why you refuse to accept the theory of the origin of species by means of physiological selection, together with the explanation which this theory affords of the continued cross-fertility of domesticated varieties? 5. Supposing this theory to be true, can you conceive of any other classes of facts which, either quantitatively or qualitatively, could more directly or more effectually prove its truth than those which have now been adduced? On these five heads I entertain no doubt. I am convinced that the theory of physiological selection is the only one that can explain the facts of inter-specific sterility on the one hand, and, on the other hand, the contrast which these facts display to the unimpaired fertility of our domesticated varieties. In conclusion, it seems desirable once more to insist that there is no antagonism or rivalry between the theories of natural and of physiological selection. For which purpose I will quote the final paragraph of my original paper.