And here we may perceive how much more defensible the new doctrine is than the old one. Even could the supporters of the Development Hypothesis merely show that the origination of species by the process of modification is conceivable, they would be in a better position than their opponents. But they can do much more than this. They can show that the process of modification has effected, and is effecting, decided changes in all organisms subject to modifying influences. Though, from the impossibility of getting at a sufficiency of facts, they are unable to trace the many phases through which any existing species has passed in arriving at its present form, or to identify the influences which caused the successive modifications; yet, they can show that any existing species--animal or vegetable--when placed under conditions different from its previous ones, _immediately begins to undergo certain changes fitting it for the new conditions_. They can show that in successive generations these changes continue; until, ultimately, the new conditions become the natural ones. They can show that in cultivated plants, in domesticated animals, and in the several races of men, such alterations have taken place. They can show that the degrees of difference so produced are often, as in dogs, greater than those on which distinctions of species are in other cases founded. They can show that it is a matter of dispute whether some of these modified forms are varieties or separate species. They can show, too, that the changes daily taking place in ourselves--the facility that attends long practice, and the loss of aptitude that begins when practice ceases--the strengthening of passions habitually gratified, and the weakening of those habitually curbed--the development of every faculty, bodily, moral, or intellectual, according to the use made of it--are all explicable on this same principle. And thus they can show that throughout all organic nature there _is_ at work a modifying influence of the kind they assign as the cause of these specific differences: an influence which, though slow in its action, does, in time, if the circumstances demand it, produce marked changes--an influence which, to all appearance, would produce in the millions of years, and under the great varieties of condition which geological records imply, any amount of change. Which, then, is the most rational hypothesis?--that of special creations which has neither a fact to support it nor is even definitely conceivable; or that of modification, which is not only definitely conceivable, but is countenanced by the habitudes of every existing organism?