_Thirdly_, each part of a tree generates itself in such a way that the maintenance of any one part depends reciprocally on the maintenance of the rest. A bud of one tree engrafted on the twig of another produces in the alien stock a plant of its own kind, and so also a scion engrafted on a foreign stem. Hence we may regard each twig or leaf of the same tree as merely engrafted or inoculated into it, and so as an independent tree attached to another and parasitically nourished by it. At the same time, while the leaves are products of the tree they also in turn give support to it; for the repeated defoliation of a tree kills it, and its growth thus depends on the action of the leaves upon the stem. The self-help of nature in case of injury in the vegetable creation, when the want of a part that is necessary for the maintenance of its neighbours is supplied by the remaining parts; and the abortions or malformations in growth, in which certain parts, on account of casual defects or hindrances, form themselves in a new way to maintain what exists, and so produce an anomalous creature, I shall only mention in passing, though they are among the most wonderful properties of organised creatures. § 65. _Things regarded as natural purposes are organised beings_ According to the character alleged in the preceding section, a thing, which, though a natural product, is to be cognised as only possible as a natural purpose, must bear itself alternately as cause and as effect. This, however, is a somewhat inexact and indeterminate expression which needs derivation from a determinate concept. Causal combination as thought merely by the Understanding is a connexion constituting an ever-progressive series (of causes and effects); and things which as effects presuppose others as causes cannot be reciprocally at the same time causes of these. This sort of causal combination we call that of effective causes (_nexus effectivus_). But on the other hand, a causal combination according to a concept of Reason (of purposes) can also be thought, which regarded as a series would lead either forwards or backwards; in this the thing that has been called the effect may with equal propriety be termed the cause of that of which it is the effect. In the practical department of human art we easily find connexions such as this; _e.g._ a house, no doubt, is the cause of the money received for rent, but also conversely the representation of this possible income was the cause of building the house. Such a causal connexion we call that of final causes (_nexus finalis_). We may perhaps suitably name the first the connexion of real causes, the second of those which are ideal; because from this nomenclature it is at once comprehended that there can be no more than these two kinds of causality.