But how apply all this to the Geology of the Old Red Sandstone? Very directly. The ichthyolites of this ancient formation hold, as has been said, an intermediate place, unoccupied among present existences, between the two series, and in some respects resemble the osseous, and in some the cartilaginous tribes. The fact reminds one of Dr. Johnson's shrewd objection to the theory embraced by Soame Jenyns in his _Free Inquiry_, and which was the theory also of Pope and Bolingbroke. The metaphysician held, with the poet and his friend, that there exists a vast and finely graduated chain of being from Infinity to nonentity--from God to nothing; and that to strike out a single link would be to mar the perfection of the whole.[Q] The moralist demonstrated, on the contrary, that this chain, in the very nature of things, must be incomplete at both ends--that between that which does, and that which does not exist, there must be an infinite difference--that the chain, therefore, cannot lay hold on _nothing_. He showed, further, that between the greatest of finite existences and the adorable Infinite there must exist another illimitable void--that the boundless and the bounded are as widely separated in their natures and qualities as the existent and the non-existent--that the chain, in short, cannot lay hold on Deity. He asserted, however, that not only is it thus incomplete at both ends, but that we must regard it as well nigh as incomplete in many of its intermediate links as at its terminal ones; that it is already a broken chain, seeing that between its various classes of existence myriads of intermediate existences might be introduced, by graduating more minutely what must necessarily be capable of infinite gradation; and that, to base an infidel theory on the supposed completeness of what is demonstrably incomplete, and on the impossibility of a gap existing in what is already filled with gaps, is just to base one absurdity on another.[R] Now, we find the Geology of what may be termed the second age of vertebrated existence (for the Lower Old Red Sandstone was such) coming curiously in to confirm the reasonings of Johnson. It shows us the greater part of the fish of an entire creation thus insinuated between two of the links of our own. [Footnote Q: "See, through this air, this ocean, and this earth, All matter quick, and bursting into birth; Above, how high progressive life may go! Around, how wide! how deep extend below! Vast chain of being! which from God began-- Nature's ethereal, human angel, man, Beast, bird, fish, insect--what no eye can see, No glass can reach; from Infinite to thee-- From thee to nothing. On superior powers Were we to press, inferior might on ours; Or in the full creation leave a void, Where, one step broken, the great scale's destroyed: From Nature's chain, whatever link you strike, Tenth, or ten thousandth, breaks the chain alike."