It is by this process alone that we can hope to make real use of analogy, without abusing it: for whilst there is danger, on the one hand, of needlessly rejecting the argument which it suggests to us, through opposite effects being observed (amongst the members of the organic world) from conditions which _we assume to be_ co-ordinate, but which in fact are not so; we may, on the other, run a similar risk (and thus fail to discern a _corresponding modus operandi_ in the maturation of like results), from a mere _à priori_ belief that the lower animals cannot be acted upon, by external influences, in a manner at all equivalent to that which is self-evident in the higher ones.
"To make a perfect observer in any department of science," writes Sir John Herschel, "an extensive acquaintance is requisite, not only with the particular science to which his observations relate, but with every branch of knowledge which may enable him to appreciate and neutralize _the effect of extraneous disturbing causes_. Thus furnished, he will be prepared to seize on any of those minute indications which often connect phænomena which seem quite remote from each other. He will have his eyes as it were opened, that they may be struck at once with any occurrence which, according to received theories, ought _not_ to happen; for these are the facts which serve as clews to new discoveries[1]."
There can be no doubt that amongst a large proportion of our naturalists, _differences_, as such, are too exclusively studied. Essential as their investigation is (for we could not progress a step without some presumptive notion as to the specific identity, or not, of the objects about which we have to treat), we should not forget that there are other questions, likewise, which ought to occupy our attention in, at any rate, an almost equal degree,--as being of eminent significance in guiding us to a correct interpretation of the phænomena with which we have to deal. Such are, more especially, similitudes and analogies, in their widest sense,--which are too often neglected, even by those who admit the necessity of recognizing them where they may be shown to exist. Lord Bacon, in referring to a similar tendency amongst a certain section of the naturalists of his day, remarks (though perhaps his love of analogies may have led him to somewhat overrate their importance): "Up to this time the industry of men has been great, and very curious in marking the variety of things, and explaining the accurate differences of animals, herbs, and fossils,--the _chief part of which_ are the mere sport of Nature, rather than serious and of use toward the sciences. Such things tend to our enjoyment, and sometimes to even practical use; but little or nothing towards an insight into Nature. And so our labour is to be turned to inquiry into, and notice of, similitudes and analogies, both in the whole and in the parts of things: for these are they which unite Nature, and begin to establish sciences[2]."