Since De Candolle’s day, it has been contended that his estimate makes the trees too old. Dr J. Lowe, in his interesting volume on _Yew-Trees_, has combated the conclusions at some length; his chapters are here freely drawn upon and compared with my own private notes. Dr Lowe’s estimate, taking young and old trees together, allows a growth of one foot in diameter for each period of 60-70 years. De Candolle’s rule would make a like growth represent 144 years at least (1 foot = 144 lines), or, supposing him to have taken the line as one-tenth of an inch--as some writers mistakenly believe[951]--120 years, or a little over. In the absence of testimony to the contrary, I think we may safely consider that De Candolle’s line was reckoned on the one-twelfth basis: indeed there appears to be no valid reason for doubting this. We note, in passing, that the calculations of Edward Jesse, the naturalist, made after measuring trees at intervals, agree closely with those of the Swiss savant[952]. Between the estimates of De Candolle and those of Dr Lowe, but far nearer to those of the latter observer, is the determination adopted by Sir R. Christison and his son, Dr D. Christison. Working on the eminently scientific method of measuring the increase of girth at a fixed point during stated periods, these observers decided that one foot for every 75 years would be more than the average rate of increase[953]. The three varying results may therefore be thus stated: De Candolle 1 foot represents 144 years The Christisons “ “ “ 75 years Dr Lowe “ “ “ 60-70, say 65 years Mr J. E. Bowman, making use of the trephine, came to the conclusion that young trees may add two lines per year, and if the soil be very rich, three lines, but that, after a diameter of two feet has been reached, De Candolle’s limit of one line yearly holds true. De Candolle’s formula, Bowman considered, “makes old trees too young, and young trees too old.” Commenting on Bowman’s mode of experiment, Dr Lowe asserts that it has “no utility whatever,” because the external rings of the tree--those reached by the trephine--are not concentric, and are not formed in the same manner as those of the young shoot. Further, Bowman’s experiments seem to have been performed on young trees only[954].