Reproduction is a fact, it is an essential property of life, and is a reality to us from observation; but irritability is not so tangible and demonstrable a property. We nevertheless may assume its universality, from the circumstance that we lose sight of it by imperceptible degrees; the irritability of the sensitive plant is as much irritability as that of the highly organized muscle; but because the faculty evades our perception, "in tapering by degrees, becoming beautifully less," we have no reason for pronouncing its total extinction at any one point of the vegetable kingdom,[11] any more than we should have {29} in saying that we see the end of the earth, when describing the extent of our vision as we stand on the sea shore. The extreme limit of our vision is the tangent of the circle in reference to our visual organs; but how many tangential points there may be beyond, it is impossible to say without knowing the dimensions of the circle. I think we are now in a condition to assume, as far as abstraction will conduct us without proceeding to an extreme length, that the _materies morbi_, or, as I will now call them for the sake of clearer distinction, _semina morbi_, possess those properties which in the abstract are common to all living beings. Another argument strikes me as capable of adding further strength to the proposition. We need but be told that a small piece of iron was placed in a certain position with regard to another piece of iron, and that the smaller piece moved through a given space and became attached to the larger, to infer that magnetic force was in operation. Supposing this magnet then to be folded in paper, and that it {30} be promiscuously placed near a compass, the deflection of the needle would indicate that some object in the vicinity was the cause of the deflection; we may farther try what positions the needle takes by varying the position of the packet, and thus point out which is the north and which the south pole of the screw of paper. If we may consider attraction then to be to gravitation what reproduction is to life, we do not err in saying in the one instance that there is a living being, and in the other there is a magnet.