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372   Life and' Letters of Francis Galton

did not see why a similar periodic anthropometric examination should not be made in all the great schools of the country; and he makes the excellent suggestion of an itinerant anthropometric laboratory, which should circulate from school to school. The idea is a very good one, and would keep the school laboratory, where such existed, up to modern standards of methods, tests and apparatus.

Discussing what should be measured, Galton observes

"One object is to ascertain what may be called the personal constants of mature life. This phrase must not be taken in too strict a sense, because there is nothing absolutely constant in a living body. Life is a condition of perpetual change. Men are about half an inch shorter when they go to bed than when they rise in the morning. Their weight is affected by diet and habit of life. All our so-called personal constants are really variables, though a large proportion of their actual variations may lie between narrow limits. Our first rule then is, that the trouble of measurement is best repaid when it is directed upon the. least variable faculties." (p. 207.)

He then touches on a point which still troubles the anthropometrician, especially in the case of mental tests

"There are many faculties that may be said to be potentially constant in adults though they are not developed, owing to want of exercise. After adequate practice, a limit of efficiency would in each case be attained and this would be the personal constant; but it is obviously impossible to guess what that constant would be from the results of a single trial. No test professes to do more than show the efficiency of the faculty at the time it was applied, and many tests do even less than this, being so novel to the person experimented on that he is maladroit, and fails to do himself justice'; consequently the results of earlier trials with ill-devised tests may differ considerably from those of later ones. The second rule then is that the actions required by the tests should be as familiar as possible'."

Galton makes a suggestion as to practice which might be worth following up, although great care would have to be taken in experimenting between sessional and secular variations. The sessional variation, or variation in a sitting (or in one closely contracted series of tests), may be largely a result of chance, partly of practice, and partly of fatigue, while the secular variation may show the marked effect of continual practice. The suggestion runs

"There is some hope that we may in time learn to eliminate the effect of an unknown amount of previous practice by three or more distinct sets of trials. There exists a rough relation between practice and proficiency which ought to be apparent wherever progress is not due to acquiring a succession of new knacks, but proceeds regularly. When no practice has previously taken place, the progressive improvement will be very rapid; then its rate will smoothly decrease until it comes to an entire stop. I suspect that a curve might be drawn between proficiency and practice, and that the data afforded by at least three successive series of tests would roughly

1 I think Galton has conceivably overlooked a point here. One test of fitness to environment is the readiness with which an individual can adapt himself to new conditions and respond promptly to the everchanging experiences of his life. In many cases therefore a novel test is a truer test of mental agility, than one which has become a familiar routine.

' This rule appears to be stated too generally; when familiarity comes in, then there will be a correlation between length of practice and efficiency, of which we do not know the intensity. When I was a boy I was taught to dovetail and ~earnt to do so creditably; any mediocre carpenter would excel my work now both in speed and neatness. On the other hand I should probably copy out far more clearly and rapidly than he would a page of letterpress. Any test to determine our relative powers of mentally controlling the rapidly moving hand ought, I think, to be made in a manner which should be as unfamiliar to us both as possible.

I The reader may consult a paper by E. S. Pearson on the "Variations in Personal Equation, etc.," Biometrika, Vol. xiv, pp. 23-102..