The discovery might be made "out of one's own head"; but here as elsewhere Layamon is a most important assistant and safeguard. A mere glance at any edition of alliterative verse, printed in half lines, will show that it has a rough resemblance on the page to octosyllabics, though the outline is more irregular. A moderately careful study of Layamon shows, as has been indicated, that, in writing this verse with new influences at work upon him, he substitutes octosyllabic couplet for it constantly. And the history in the same way shows that this occasional substitution became a habitual one with others. Not that there is any mystical virtue in four feet, despite their frequency in the actual creation: but that, as an equivalent of the old half line, the choice lies practically between three and four. Now a three-foot line, though actually tried as in the _Bestiary_ and in parts of _Horn_, is, as a general norm, too short, is ineffective and jingly, brings the rhyme too quick, and hampers the exhibition of the sense by a too staccato and piecemeal presentment. The abundant adoption of the octosyllable in French no doubt assisted the spread in English. But it is not unimportant to observe that English translators and adapters of French octosyllabic poems by no means always preserve the metre, and that English octosyllables often represent French poems which are differently metred in the original.
IV. DECASYLLABLE.--A connected literary origin for this great line--the ancient staple of French poetry, the modern staple of English, and (in still greater modernity) of German to some extent, as well as (with the extension of one syllable necessitated by the prevailing rhythm of the language) of Italian throughout its history--has always been found extraordinarily difficult to assign. That some have even been driven to the line which furnishes the opening couplet of the Alcaic
an invariably _hen_decasyllabic line of the most opposite rhythm, constitution, and division, will show the straits which must have oppressed them. The fact is that there is nothing, either in Greek or Latin prosody, in the least resembling it or suggestive of it. To connect it with these prosodies at all reasonably, it would be necessary to content ourselves with the supposition, not illogical or impossible, but not very explanatory, that somebody found the iambic dimeter too short, and the iambic trimeter too long, and split the difference.