The line ABC records, by its distance at successive points from the line AC, the number of recruits reaching successive inches of height. It shows, e.g. (as indicated by the dotted lines) that the number of recruits between 5 ft. 11 in. and 6 ft. was about 1500, and the number of those between 5 ft. 7 in. and 5 ft. 8 in. about 4000.[[40]]
Such figures, when they simply record the results of the fact that the likeness of the offspring to the parent in evolution is constantly inexact, are (like the records of other cases of 'chance' variation) fairly symmetrical, the greatest number of instances being found at the mean, and the descending curves of those above and those below the mean corresponding pretty closely with each other. Boot manufacturers, as the result of experience, construct in effect such a curve, making a large number of boots of the sizes which in length or breadth are near the mean, and a symmetrically diminishing number of the sizes above and below it.
In the next chapter I shall deal with the use in reasoning of such curves, either actually 'plotted' or roughly imagined. In this chapter I point out, firstly, that they can be easily remembered (partly because our visual memory is extremely retentive of the image made by a black line on a white surface) and that we can in consequence carry in our minds the quantitative facts as to a number of variations enormously beyond the possibility of memory if they were treated as isolated instances; and secondly, that we can by imagining such curves form a roughly accurate idea of the character of the variations to be expected as to any inherited quality among groups of individuals not yet born or not yet measured.
The third and last division under which knowledge of man can be arranged for the purposes of political study consists of the facts of man's environment, and of the effect of environment upon his character and actions. It is the extreme instability and uncertainty of this element which constitutes the special difficulty of politics. The human type and the quantitative distribution of its variations are for the politician, who deals with a few generations only, practically permanent. Man's environment changes with ever-increasing rapidity. The inherited nature of every human being varies indeed from that of every other, but the relative frequency of the most important variations can be forecasted for each generation. The difference, on the other hand, between one man's environment and that of other men can be arranged on no curve and remembered or forecasted by no expedient. Buckle, it is true, attempted to explain the present and prophesy the future intellectual history of modern nations by the help of a few generalisations as to the effect of that small fraction of their environment which consisted of climate. But Buckle failed, and no one has attacked the problem again with anything like his confidence.
We can, of course, see that in the environment of any nation or class at any given time there are some facts which constitute for all its members a common experience, and therefore a common influence. Climate is such a fact, or the discovery of America, or the invention of printing, or the rates of wages and prices. All nonconformists are influenced by their memory of certain facts of which very few churchmen are aware, and all Irishmen by facts which most Englishmen try to forget. The student of politics must therefore read history, and particularly the history of those events and habits of thought in the immediate past which are likely to influence the generation in which he will work. But he must constantly be on his guard against the expectation that his reading will give him much power of accurate forecast. Where history shows him that such and such an experiment has succeeded or failed he must always attempt to ascertain how far success or failure was due to facts of the human type, which he may assume to have persisted into his own time, and how far to facts of environment. When he can show that failure was due to the ignoring of some fact of the type and can state definitely what that fact is, he will be able to attach a real meaning to the repeated and unheeded maxims by which the elder members of any generation warn the younger that their ideas are 'against human nature.' But if it is possible that the cause was one of mental environment, that is to say, of habit or tradition, or memory, he should be constantly on his guard against generalisations about national or racial 'character.'
One of the most fertile sources of error in modern political thinking consists, indeed, in the ascription to collective habit of that comparative permanence which only belongs to biological inheritance. A whole science can be based upon easy generalisations about Celts and Teutons, or about East and West, and the facts from which the generalisations are drawn may all disappear in a generation. National habits used to change slowly in the past, because new methods of life were seldom invented and only gradually introduced, and because the means of communicating ideas between man and man or nation and nation were extremely imperfect; so that a true statement about a national habit might, and probably would, remain true for centuries. But now an invention which may produce profound changes in social or industrial life is as likely to be taken up with enthusiasm in some country on the other side of the globe as in the place of its origin. A statesman who has anything important to say says it to an audience of five hundred millions next morning, and great events like the Battle of the Sea of Japan begin to produce their effects thousands of miles off within a few hours of their happening. Enough has already occurred under these new conditions to show that the unchanging East may to-morrow enter upon a period of revolution, and that English indifference to ideas or French military ambition are habits which, under a sufficiently extended stimulus, nations can shake off as completely as can individual men.