cause of heat, and heat, again, is the cause of the renewed occurrence of the chemical combination. But this is nothing more than a chain of causes and effects, the links of which have alternately the same name. The burning, A., produces free heat, B., this produces new burning, C. (i.e., a new effect which has the same name as the cause A., but is not individually identical with it), this produces new heat, D. (which is not really identical with the effect B., but only according to the concept, i.e., it has the same name), and so on indefinitely. A good example of what in ordinary life is called reciprocity is afforded by a theory about deserts given by Humboldt (Ansichten der Natur, 2d ed., vol. ii. p. 79). In the sandy deserts it does not rain, but it rains upon the wooded mountains surrounding them. The cause is not the attraction of the clouds by the mountains; but it is the column of heated air rising from the sandy plain which prevents the particles of vapour from condensing, and drives the clouds high into the heavens. On the mountains the perpendicular rising stream of air is weaker, the clouds descend, and the rainfall ensues in the cooler air. Thus, want of rain and the absence of plants in the desert stand in the relation of reciprocity; it does not rain because the heated sand-plain sends out more heat; the desert does not become a steppe or prairie because it does not rain. But clearly we have here again, as in the example given above, only a succession of causes and effects of the same names, and throughout nothing essentially different from simple causality. This is also the case with the swinging of the pendulum, and indeed also with the self-conservation of the organised body, in which case likewise every state introduces a new one, which is of the same kind as that by which it was itself brought about, but individually is new. Only here the matter is complicated, because the chain no longer consists of links of two kinds, but of many kinds, so that a link of the same name only recurs after several others have intervened. But we
