That the essential identity of bodily organs and tools is no ingenious paradoxical way of putting things is evident from the fact that we speak of bodily organs at all.  Organ means tool.  There is nothing which reveals our most genuine opinions to us so unerringly as our habitual and unguarded expressions, and in the case under consideration so completely do we instinctively recognise the underlying identity of tools and limbs, that scientific men use the word “organ” for any part of the body that discharges a function, practically to the exclusion of any other term.  Of course, however, the above contention as to the essential identity of tools and organs does not involve a denial of their obvious superficial differences—­differences so many and so great as to justify our classing them in distinct categories so long as we have regard to the daily purposes of life without looking at remoter ones.

If the above be admitted, we can reply to those who in an earlier chapter objected to our saying that if Mr. Darwin denied design in the eye he should deny it in the burglar’s jemmy also.  For if bodily and non-bodily organs are essentially one in kind, being each of them both living and non-living, and each of them only a higher development of principles already admitted and largely acted on in the other, then the method of procedure observable in the evolution of the organs whose history is within our ken should throw light upon the evolution of that whose history goes back into so dim a past that we can only know it by way of inference.  In the absence of any show of reason to the contrary we should argue from the known to the unknown, and presume that even as our non-bodily organs originated and were developed through gradual accumulation of design, effort, and contrivance guided by experience, so also must our bodily organs have been, in spite of the fact that the contrivance has been, as it were, denuded of external evidences in the course of long time.  This at least is the most obvious inference to draw; the burden of proof should rest not with those who uphold function as the most important means of organic modification, but with those who impugn it; it is hardly necessary, however, to say that Mr. Darwin never attempted to impugn by way of argument the conclusions either of his grandfather or of Lamarck.  He waved them both aside in one or two short semi-contemptuous sentences, and said no more about them—­not, at least, until late in life he wrote his “Erasmus Darwin,” and even then his remarks were purely biographical; he did not say one syllable by way of refutation, or even of explanation.