Now, in view of these results the question arises,—Why should the sexual methods of propagation have become so general, if their effect has been that of determining the necessary death of all individuals presenting them? Why, in the course of organic evolution, should these newer methods have been imposed on all the higher organisms, when the consequence is that all these higher organisms must pay for the innovation with their lives? Weismann’s answer to this question is as interesting and ingenious as all that has gone before. Seeing that sexual propagation is so general as to be practically universal among multicellular organisms, it is obvious that in some way or another it must have had a most important part to play in the general scheme of organic evolution. What, then, is the part that it does play? What is its _raison d’être_? Briefly, according to Weismann, its function is that of furnishing congenital variations to the ever-watchful agency of natural selection, in order that natural selection may always preserve the most favourable, and pass them on to the next generation by heredity. That sexual propagation is well calculated to furnish congenital variations may easily be rendered apparent. We have only to remember that at each union there is a mixture of two sets of germinal elements; that each of these was in turn the product of two other sets in the preceding generation, and so backwards _ad infinitum_ in an ever doubling ratio. Remembering this, it follows that the germinal elements of no one member of a species can ever be the same as those of any other member born of different parents; on the contrary, while both are enormously complex products, each has had a different ancestral history, such that while one presents the congenital admixtures of thousands of individuals in one line of descent, the other presents similar admixtures of thousands of other individuals in a different line of descent. Consequently, when in any sexual union two of these enormously complex germinal elements fuse together, and constitute a new individual out of their joint endowments, it is perfectly certain that that individual cannot be exactly like any other individual of the same species which has been born of different parents. The chances must be infinity to one against any single mass of germ-plasm being exactly like any other mass of germ-plasm; while any amount of latitude as to difference is allowed, up to the point at which the difference becomes too pronounced to satisfy the conditions of fertilization—in which case, of course, no new individual is born. Hence, theoretically, we have here a sufficient cause for all individual variations of a congenital kind that can possibly occur within the limits of fertility, and, therefore, that can ever become actual in living organisms. In point of fact, Weismann believes—or, at any rate, provisionally maintains—that this is the sole and only cause of