We see a machine as a whole, we call it by a name and individualize it; we look at our own limbs, and know that the combination forms an individual which springs from a single centre of reproductive action; we therefore assume that there can be no reproductive action which does not arise from a single center; but this assumption is unscientific, and the bare fact that no vapour engine was ever made entirely by another, or two others, of its own kind, is not sufficient to warrant us in saying that vapour engines have no reproductive system.