Having distinguished two primary explanatory models, mechanism and agency, I want to try to apply that distinction in an example -- and the one I have in mind is one that excited much interest over a century and a half ago: the phenomenon of species change.
That the phenomenon existed at all was by no means evident, in the first place. Species appeared to be not just fixed, but admirably and ingeniously suited to their place in the world, which of course is just what one would expect from an Intelligent Designer. Thus, like contemporary ID advocates, the explanation for the remarkable "fitness" of species is modeled on the notion of the will or purpose of a supernatural agent, and that was all one could (or needed to) say. The fossils turned up by geologists (later, paleontologists), however, began to put this neat and static picture under increasing stress, and the distasteful but unavoidable conclusion seemed to be that species that once existed no longer do, and species that now exist once did not. What could account for this?
Jean-Baptiste Lamarck, as everyone knows, tried to answer that. He thought that individual organisms tried to perform certain actions in response to changing conditions, for example; that in doing so changed themselves, in the way that exercise develops muscle; and that these changes were then inherited by subsequent generations, which over time resulted in a changed species. Now, as everyone also knows, Lamarck was mistaken about the heritability of acquired characteristics, and this mistake is often taken as the defining error of Lamarckism. But I think that mistake was a relatively superficial one, and even somewhat understandable -- the deeper, more profound problem with Lamarckism has to do with how change comes about in the first place. It is true, of course, that some organs (mainly muscle) are developed through use, but many changes to species -- e.g., color of fur, structure of eye -- have nothing to do with anything that an organism is or could be "trying" to do. Moreover, even if an organism could try to change the color of its fur or the structure of its eyes, how would it know that that's the right thing to do -- that is, how would it know how to design itself so as to fit its environment? The primary problem with Lamarckism, in other words, isn't the notion of the heritability of acquired characteristics, but the fact that it lacks amechanism for adaptational change -- instead, it simply located the agentof biological fitness within the organism itself, where it remained as mysterious as the ways of the supernatural Designer.
And then along came Darwin. By and large, he understood that acquired characteristics (which would be at least as likely to be damaging as beneficial) were not inherited, but that was not the essence of his surprisingly simple and remarkably general insight. What Darwin provided was precisely what Lamarck lacked -- a mechanism for adaptational change. It broke the process into two parts or aspects -- on the one hand, there was a blind but incessant generation of small random (i.e., not willed or agent-directed) changes; and on the other hand, there was an equally blind (again, no agent involvement), equally incessant process of culling those random changes. "Natural selection", as a phrase, emphasizes the agent-less nature of that culling process just by its contrast with the agent-based process that goes on in human or "domestic" selection. And with that two-part mechanism, what had seemed vague, obscure, and mysterious, suddenly became lucid and clear -- and perhaps all the more marvelous (and controversial) just because of that clarity.
This contrast between mechanism and agency is so important, I think, that it really has become implicit in the very meaning of evolutionary change. It's worth observing, after all, that the sort of willed or agent-directed change that Lamarck thought was at the basis of species change does indeed occur (even though he was wrong about its heritability), but is inherently limited -- we could think of it as exhibiting the potential of a species at a point in time. Evolutionary change, on the other hand, represents a change in that potential, and that is something that nature or the environment imposes, through the twin but independent mechanisms of natural variation and selection.