The original Darwinian theory was soon found to be altogether insufficient to account for the observed facts, because of the tendency of the bare struggle for existence to produce degradation rather than elevation; because of the testimony of geology to the fact that introduction of new species takes place in times of expansion rather than of struggle; because of the manifest tendency of the breeds produced by artificial selection to become infertile and die out in proportion to their deviation from the original types; and because of the difficulty of preventing such breeds from reverting to the original forms, which seem in all cases to be perfectly equilibrated in their own parts and adapted to external nature, so that varieties tend, as if by gravitative law, to fall back into the original moulds. A great variety of other considerations--as those of sexual selection, reproductive acceleration and retardation, periods of more and less rapid evolution, innate tendency to vary at particular times and in particular circumstances--have been imported into the original doctrine. Thus the original Darwinism is a thing of the past, even in the mind of its great author, though it has proved the fruitful parent of a manifold progeny of allied ideas which continue to bear its name. In this respect Darwinism is itself amenable to the law of evolution, and has been continually changing its form under the influence of the controversial struggles which have risen around it. Darwinism was not necessarily atheistic or agnostic. Its author was content to assume a few living beings or independent forms to begin with, and did not propose to obtain them by any spontaneous action of dead matter, nor to account for the primary origin of life, still less of all material things. In this he was sufficiently humble and honest; but the logical weakness of his position was at once apparent. If creation was needed to give a few initial types, it might have produced others also. The followers of Darwin, therefore, more especially in Germany, at once pushed the doctrine back into Agnosticism and Monism, giving to it a greater logical consistency, but bringing it into violent conflict with theism and with common sense.