The Greek word for end is telos, source of the term "teleology," so with this type of cause we come to the thorny question of teleology in nature. Since the time of Aristotle it has been almost axiomatic that nature acts for an end. If evolution is viewed as a natural process, does this also entail that it is a teleological process, one that is goal-directed, in the sense that higher and more developed species are not merely the result of chance but were somehow determined in advance? There is no easy answer to this question, but its very asking enables one to reflect on ends or final causes and how these may be operative within the world of nature.

It can be helpful here to distinguish three different meanings of the word "end," not all of which are equally identifiable in natural processes. The first and simplest meaning is that of end in the sense of terminus, the point at which a process stops. In a journey from New York to Washington, the nation's capital is the end of the trip, the place where the traveler comes to rest. Similarly the natural fall of an object terminates when the heavy body either reaches the center of gravity to which it is tending or encounters some obstacle that impedes its motion and brings it to rest. A plant grows from a seedling to full maturity, at which point it stops growing; the same could be said of the developmental process of a flea or an elephant. Natures are stable kinds, that is, within a certain range they represent regions of stability in a world of flux. In our experience fleas do not grow and grow indefinitely, say, until they reach the size of elephants, nor are elephants found as small as fleas. Growth processes terminate; to the extent that these are natural processes, the states at which they terminate are ends reached by nature and so satisfy the first meaning of final cause. . . .

The second meaning of end or goal adds to the idea of terminus the notion that it is somehow a perfection or good attained through the process. . . . In cases of organic growth the end product represents a superior grade of being over the stage at which it began. It is also more perfect, in the etymological sense of per-factum, as that which is thoroughly made and possesses no de-factum, i.e., is lacking in nothing it should possess as a member of its species. . . .

The third meaning of end is more specialized still, for it adds to the notion of termination and perfection that of intention or aim. This serves to identify the type of final causality found in cognitive agents. Animals and humans are natural agents of this type: many of their activities are planned or intended in advance and so can be seen as end-directed from their beginning. A person building a house or a bird a nest must have in advance some notion of what is intended, for otherwise neither builder would know how to gather the materials. There seems to be a difference in the two cases, however, for the bird does its work by instinct and tends to make the nest in a form that is predetermined by its species, whereas man is not so limited and can generate the multiplicity of dwellings recorded in human history.

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Selection from William A. Wallace, O.P., The Modeling of Nature

(Washington, DC: The Catholic University of America Press, 1996), 15-17.