the proof of design or final cause in A and D, whatever it amounted
to, at all weakened by the discovered intermediate forms? Rather does
not the proof extend to the intermediate species, and go to show that
all four were equally designed? Suppose, now, the number of
intermediate forms to be much increased, and therefore the gradations
to be closer yet, as close as those between the various sorts of dogs,
or races of men, or of horned cattle: would the evidence of design, as
shown in the structure of any of the members of the series, be any
weaker than it was in the case of A and D? Whoever contends that it
would be should likewise maintain that the origination of individuals
by generation is incompatible with design, and so take a consistent
atheistical view of Nature. Perhaps we might all have confidently
thought so, antecedently to experience of the fact of reproduction.
Let our experience teach us wisdom.

These illustrations make it clear that the evidence of design from
structure and adaptation is furnished complete by the individual
animal or plant itself, and that our knowledge or our ignorance of the
history of its formation or mode of production adds nothing to it and
takes nothing away. We infer design from certain arrangements and
results; and we have no other way of ascertaining it. Testimony,
unless infallible, cannot prove it, and is out of the question here.
Testimony is not the appropriate proof of design: adaptation to
purpose is. Some arrangements in Nature appear to be contrivances, but
may leave us in doubt. Many others, of which the eye and the hand are
notable examples, compel belief with a force not appreciably short of
demonstration. Clearly to settle that these must have been designed
goes far towards proving that other organs and other seemingly less
explicit adaptations in Nature must also have been designed, and
clinches our belief, from manifold considerations, that all Nature is