Again, why should the physiological change be thus associated with _every conceivable variety_ of morphological change? Throughout the length and breadth of both vegetable and animal kingdoms we find this association, in the great majority of cases, where new species arise. Therefore, on the supposition that in all such cases the physiological change has been adventitiously induced by the morphological changes, we have to face an apparently unanswerable question--Why should the reproductive mechanism of all organic beings have been thus arranged, as it were, to change in immediate response to the very slightest alteration in the complex harmony of "somatic" processes, which now more than ever is recognized as exercising so comparatively little influence on the _hereditary_ endowments of this mechanism? Consider the difference between a worm and the bird that is eating it, an oak tree and the gall-insect that is piercing it: are we to suppose that in all cases, no matter how greatly the types differ, they must agree in this, that when any parts of these complex structures change, ever so slightly, the reproductive system is almost certain to be adventitiously affected, yet always thus affected in the same peculiar way? If it be answered that the reproductive system is known to be very sensitive to slight changes in the external conditions of life, the answer proves too much. For though this is true, yet our opponents must acknowledge that the reproductive system is not so sensitive, _in this particular respect_, as their interpretation of the origin of specific infertility requires. The proof of this point is overwhelming, for there is the evidence from the entire range of our domesticated productions, both vegetable and animal. Here the amount of structural change, which has been slowly accumulated by artificial selection, is often much greater in amount, and incomparably more rapid, than that which has been induced between allied species by natural selection; and yet there is scarcely any indication of the reproductive system having been affected in the particular way that our opponents' theory requires. There are many instances of its having been affected in sundry other ways (chiefly, however, without any accompanying morphological change); but among all the thousands of our more or less enormously modified artificial types, there is scarcely one instance of such a peculiar sexual relation between the modified descendants of a common type as so usually obtains between allied species in nature. Yet in all other respects evolutionists are bound to believe that the process of modification has been in both cases strictly analogous. Why then this conspicuous difference with respect to the reproductive system?