For this act was carried through many species which had no need of it to multiply by means of proliferation; until a moment came when the non-sexually produced offspring found themselves once more constrained to a renewal of the copulative function, and the circle came full. Such was the multiple state of life, sprung from the union of two parent cells, the association of many non-sexually originated generations of cell units; its growth meant their increase, and the generative circle came full again when sex-cells, specially developed elements for the purpose of reproduction, had established themselves and found the way to a new mingling that drove life on afresh.