To be designed by an intelligence is a special case of depending on an intelligence: everything that is so designed is so dependent, but not everything that is dependent on intelligence is designed by it. To say that some organism or system is designed by an intelligence means that everything in it can be related to the function of the system, and so we can give evidence against design by finding features in the system that are vestigal or otherwise somehow unrelated to the system. Here we might notice the distinction between design and adaptation: my kinds, for example, might wonder why anyone designed the plug for the DVD charger to look so strange, or why it never crossed anyone’s mind to just put standard sockets in a car. So long as they were determined to see this plug-socket relation as arising from design, they might hypothesize that there was something about the nature of cars that called for a larger, inch-round socket, and that it would be somehow inappropriate to put a merely standard socket in the car. Maybe the kids would think that the designers made the socket bigger so that it would be easier to find when we fumbled around for it while driving; maybe they would advance a Aristotelian theory that circular plugs are inherently more perfect than standard sockets, etc. For anyone over thirty, the reason why this is a pointless line of analysis is clear. No one designed the plug-socket connection but simply adapted the plug to something that was not even a socket at all but a cigarette lighter.
Design thus calls for a certain analysis of the parts of systems that is refuted by finding vestigal, junk, or merely adaptive parts in the system. But even if design is the clearest indicator of intelligence being involved with a system, it is not the only indicator. This is clear even from the examples we’ve used to illustrate the difference: to say that the DVD plug-socket system was designed system is a dead end, but it is equally a dead end to deny that it depended on intelligence to come to be. There are even obvious elements of design in the system, but just not in way that can be explained in relation to the ideal functionality of the whole system.
The crucial difference between design and dependence is the role allowed for secondary causes and historical existence. Adaptation means that the one adapting recognizes that the thing he is adapting has its own causal structure apart from his purposes, and that the whole of its being is not ordered to the singular end that he puts it to. That said, when things have structures adapted to certain ends, or even designed for certain ends, the time can come when those ends are no longer important or even desirable. Yet the structure remains, and so it must either be adapted to new goals, or carried along as vestigal, or become a liability. Ideally, of course, we would want it to be adapted to the new goal. This is, at least, how we would design things to happen.
It is possible that adaptation presupposes design, and/or that all that appears vestigal or junk serves some useful, designed purpose. We can only wait and see to what extent the hypothesis that nature does nothing in vain is empirically useful. At the moment, it seems true to some extent, but clearly limited. But whatever we end up thinking about design, the question of dependence on intelligence is still a live issue, though it is not necessarily one that we can arbitrate by empirical or positive methods.