In this sense it may well be observed that the organismismoresen$itive or more irritable, or has a greater reproductive capacity than another, just as werpay observe that the sensibility of one is different in kind from that ofanother, that one reacts differently to a given stimulus than another, eg a horse reacts differently to oats than to hay, and a dog againdifEerently to both, differences as readily observable as that one body is harder than another, and so on.