The ‘final cause’ or ‘purpose’, i.e., the adaptation of the parts to the requirements of the whole, becomes a necessary part of the explanation of why structures of the kind exist: we are bound to explain the fact that the elements behave in a certain way by the circumstance that this sort of conduct is most likely to preserve the whole—on the preservation of which depends the preservation of the individuals, which would therefore not exist if they did not behave in this manner. A ‘teleological’ explanation is thus entirely in order so long as it does not imply design by a maker but merely the recognition that the kind of structure would not have perpetuated itself if it did not act in a manner likely to produce certain effects, and that it has evolved through those prevailing at each stage who did.
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