The man of various pursuits does not, in any case, keep them up simultaneously; he is led by inclination or compelled by necessity to give predominance to one or another. If you have fifteen different pursuits, ten of them, at any given time, will be lying by neglected. The metaphor commonly used in reference to neglected pursuits is borrowed from the oxidation of metal; it is said that they become rusty. This metaphor is too mild to be exact. Rust on metal, even on polished steel, is easily guarded against by care, and a gun or a knife does not need to be constantly used to keep it from being pitted. The gunsmith and the cutler know how to keep these things, in great quantity, without using them at all. But no one can retain knowledge without using it. The metaphor fails still more seriously in perpetuating a false conception of the deterioration of knowledge through neglect. It is not simply a loss of polish which takes place, not a loss of mere surface-beauty, but absolute disorganization, like the disorganization of a carriage when the axle-tree is taken away. A rusty thing may still be used, but a disorganized thing cannot be used until the lost organ has been replaced. There is no equivalent, amongst ordinary material losses, to the intellectual loss that we incur by ceasing from a pursuit. But we may consider neglect as an enemy who carries away the girths from our saddles, the bits from our bridles, the oars from our boats, and one wheel from each of our carriages, leaving us indeed still nominally possessors of all these aids to locomotion, but practically in the same position as if we were entirely without them. And as an enemy counts upon the delays caused by these vexations to execute his designs whilst we are helpless, so whilst we are laboring to replace the lost parts of our knowledge the occasion slips by when we most need it. The only knowledge which is available when it is wanted is that which we habitually use. Studies which from their nature cannot be commonly used are always retained with great difficulty. The study of anatomy is perhaps the best instance of this; every one who has attempted it knows with what difficulty it is kept by the memory. Anatomists say that it has to be learned and forgotten six times before it can be counted as a possession. This is because anatomy lies so much outside of what is needed for ordinary life that very few people are ever called upon to use it except during the hours when they are actually studying it. The few who need it every day remember is as easily as a man remembers the language of the country which he inhabits. The workmen in the establishment at Saint Aubin d'Écroville, where Dr. Auzoux manufactures his wonderful anatomical models, are as familiar with anatomy as a painter is with the colors on his palette. _They_ never forget it. _Their_ knowledge is never made practically valueless by some yawning hiatus, causing temporary incompetence and delay.