More permanent alterations are produced by cuts or by wounds that have healed and left a white scar. An instance of this is seen in D in the plate (p. 66), which represents a print of the left-hand thumb of the present writer. Running across the ridges, and breaking their continuity is a line which marks the place where twenty years ago the slip of a knife nearly severed a piece from the thumb. The effect of this cut has been to add a fresh feature of identity to those furnished by the original ridges, without interfering with the identification of the latter. In the case of jagged cuts or of scars formed in the healing of an ulcer the ridges may be so distorted as to be practically indistinguishable in that place, or they may even be entirely obliterated. Old age has also an obliterating effect upon the ridges, so much so that the finger-prints of an old man frequently exhibit transverse white markings, indicating signs of the surface disintegration of the skin. A most important point in the application of finger-prints to the identification of the individual is the persistence of the main details throughout life, since otherwise much of the value of the method would be lost. The observations made by Sir William Herschel in India showed that after the lapse of twenty years there was so little change in the finger-prints of a large number of persons that they could still readily be identified in this way. Sir Francis Galton has also proved the persistence of the general peculiarities in the prints for periods of over thirty years. He points out that an exact correspondence as to the minutiæ is not always to be expected, since what appears to be a ridge in one print may be really the result of imperfect printing of an enclosure. Apart from possible imperfections in the method, there is also a possibility of variation due to the effect of age rendering the ridges less continuous. In one of the examples given by him finger-prints were taken of a child of two and a half in 1877, and again thirteen years later. Between two of the prints there were forty-two points of resemblance and only one point of difference. This was a small forked ridge which appeared in the print of the baby, but had been filled up in the print from the boy. This instance is mentioned as unique, for in every other case examined by Sir Francis Galton, comparing prints of the boy with the man, and the man with the old man there was perfect correspondence between the selected points. He therefore concludes that "we are justified in inferring that between birth and death there is absolutely no change in, say, 699 out of 700 of the numerous characteristics in the markings of the same person such as can be impressed by them whenever it is desirable to do so."