His notes were subjected to perpetual revisions and additions.
Recensions and revisions were twice made, in 1368 and 1531; but how far the true Ibelin was recovered, and what additions or alterations were made at these two dates, we cannot tell.
Those who work with living forms of which it is possible to obtain a large number of specimens, and those who make revisions of the provisional species of palaeontologists, are slowly coming to some such conception as that a species is the abstract central point around which a group of variations oscillate, and that the peripheral oscillations of one species may even overlap those of an allied species.
Several revisions of the text exist, the later of which present such striking agreement with the later Wycliffite version that we shall not be far wrong if we assume that they were made use of to a considerable extent by the revisers of this version.
Besides the attainments mentioned above, in respect of operative progress, many important revisions of older rule-of-thumb knowledge have come about, and not a few other substantial discoveries.
