The facts which are relied on in support of the idea of 'fixity of species' show, at any rate, that a given superiority will remain stationary for thousands of years; and no one supposes that the progeny of an organism will vary unless some external or internal cause of variation accompanies the inheritance. Mr. Darwin agrees with Mr. Spencer in admitting the difficulty of distinguishing between the effects of some definite action of external conditions, and the accumulation through natural selection of inherited variations serviceable to the organism. But even in cases where the distinction could be clearly established, I think we should only see an _historical_ distinction, that is to say, one between effects produced by particular causes now in operation and effects produced by very complex and obscure causes in operation during ancestral development.... Natural Selection is only the expression of the results of obscure physiological processes." The last statement is one to which Darwin himself would certainly not have objected. It is an extension of the principle implicitly involved in all his work and explicitly stated in his later work, although the chief emphasis is laid on outer conditions. The extension of the idea of competition from the outer condition of organisms to the more ultimate physiological unities of organ and tissue is a philosophic gain. It is evident, however, that that for which Darwin is seeking is not a philosophical generalization which shall include outer and inner change under one highest law, but, first of all, the particular causes of particular variation interesting to the specialist in biology. It is made too clear for mistake in "The Variation of Animals and Plants under Domestication" that the uncertainty with regard to such particular forms of cause is the spring of his declaration of our ignorance as to variation. The possibility of an inclusion of lower in higher generalizations he would not deny; though the special laws first occupy his attention. Doubtless, his work is not, as is no man's, wholly free from inconsistencies and contradictions,--which are due, in part, to the fact that every scientific theory is, even in the thought of the individual, an evolution. But the declaration of mystery in the question of variation is not equivalent to a theory of accident, of transcendental mystery, or of some special organic or vital force, such as Claude Bernard especially opposed; it is merely and simply a statement of the mystery of present ignorance. This fact is expressly stated in Darwin's later work. We find, for instance, in the introduction to a letter to the editor of "Nature," written in 1873, the origin of many instincts referred to "modifications or variations in the brain, which we, in our ignorance, most improperly call spontaneous or accidental;" and we have, in "The Variation of Animals and Plants under Domestication,"[98] such passages as the following: "When we reflect on