"Biologists must constantly keep in mind that what they see was not designed, but rather evolved."

A helpful reminder from Nobel laureate Francis Crick, in What Mad Pursuit, p. 138 (1990):

Biologists must constantly keep in mind that what they see was not designed, but rather evolved.

Of course, the presumption underlying his exhortation that living things were not designed is philosophical and metaphysical, not scientific.

Longer quote:

Biologists must constantly keep in mind that what they see was not designed, but rather evolved. It might be thought, therefore, that evolutionary arguments would play a large part in guiding biological research, but this is far from the case. It is difficult enough to study what is happening now. To figure out exactly what happened in evolution is even more difficult. Thus evolutionary achievements can be used as hints to suggest possible lines of research, but it is highly dangerous to trust them too much. It is all too easy to make mistaken inferences unless the process involved is already very well understood.