the individual differences between organic beings in a state of nature, as shown by every wild animal knowing its mate; and when we reflect on the infinite diversity of many varieties of our domesticated productions, we may well be inclined to exclaim, though falsely, as I believe, that variability must be looked at as an ultimate fact, necessarily contingent on reproduction. Those authors who adopt this latter view would probably deny that each separate variation has its own proper exciting cause. Although we can seldom trace the precise relation between cause and effect, yet the considerations presently to be given lead to the conclusion that each modification must have its own distinct cause." It is "probable that variability of every kind is directly or indirectly caused by changed conditions of life. Or, to put the case under another point of view, if it were possible to expose all the individuals of a species during many generations to absolutely uniform conditions of life, there would be no variability.... The causes which induce variability act on the mature organism, on the embryo, and, as we have good reason to believe, on both sexual elements before impregnation has been effected." Darwin further considers, in this same book, some of the probable particular causes of variation, as given in climate and food. And it may be remarked, in this connection, that Rolph's criticism of the impossibility of progress under conditions of want is irrelevant as applied to Darwin, since the latter himself says expressly: "Of all causes which induce variability, excess of food, whether or not changed in nature, is probably the most powerful";[99] again: "We have reason to suspect that an habitual excess of highly nutritive food, or an excess relatively to the wear and tear of the organization from exercise, is a powerful exciting cause of variability."[100] Rolph's criticism is probably due to forgetfulness of the fact that Darwin limited the struggle for existence to that of complete organisms with one another, and that, under such a limitation of the conception to external struggle, a condition of want cannot be conceived as necessarily precluding a monopoly of abundance by best-fitted individuals. Theories with regard to the special outer causes and resulting physiological conditions of variation have been gradually added to, as facts on this score have accumulated. But, as investigation advances, the question is seen to involve all the problems of the intricate chemical and mechanical nature of physiological structure in its manifold forms and degrees of organization. The field stretches out in this direction, under our contemplation, to an indefinite distance; and science appears as yet to have passed only the outer limits of its territory.