Sorry, not following you. Can you expand or clarify?
The adaptation of species to their environment does at least look as though an intelligence designed them to fit their environment; whereas although you
can always imagine a thunder-god being responsible for the lightning, it doesn't particularly look as though it's intelligently directed --- lightning does not, for example, exclusively smite sinners, or we might begin to wonder if it was intelligently directed.
So although it was always
possible to proffer "goddidit" as an explanation for anything you didn't understand, only adaptation seemed to
require such an explanation. In that case the supposition of an intelligent motivating force was not merely an argument from ignorance --- it was, at the very least, an argument from analogy.
So although people did ascribe all sorts of natural phenomena to the gods, there was this one case --- adaptation --- where doing so was not completely arbitrary and stupid. And then along came Darwin and Wallace with a better explanation, and suddenly the only remotely
good reason (from a scientific standpoint) for believing in an invisible anthropomorphic entity was without merit.