Karl Pearson 
271 
Illustration I, and a general or average decadence greater by 25 per cent. It 
would be idle to assert that one or other corresponds to a greater racial degeneracy 
until we have settled whether it is more important to a nation to have in the case 
of a desirable character (i) a high average value, (ii) a larger percentage of note- 
worthy individuals, or (iii) a smaller percentage of unfit individuals. We do not 
know at present what relative weight is to be given to these categories ; but we 
can see that in any quantitative theory of the socially desirable characters in a 
community, they naturally arise from the analysis of a differential fertility. Such 
factors as dearth of leaders, lowered general intelligence of the community, multi- 
plication of the unfit, are seen, even by a preliminary analysis of this kind, not 
to be unrelated causes, by aid of which the historian accounts in a vague manner 
for racial and political changes. They are associated factors, explicable and 
measurable when we know the outlines of a theory of differential fertility. 
No special stress must be laid on the particular numbers chosen above to 
illustrate this discussion. They are round numbers not wholly inappropriate, 
perhaps, to what we know of intelligence and fertility in man. They are used to 
suggest the main outlines of a discussion of such problems. Considering the 
immense masses of statistical, especially demographic data now being accumulated 
in all civilised countries, it seems to me that the historian of the future will have 
the means of testing in a way, never yet feasible, the changing fitness or unfitness 
of nations. In that future absolute and differential fertility, the abundance or 
dearth of leaders, the average intelligence of a nation, the reduction or multiplication 
of its unfit, will be recognised as the basal factors in racial progress, the biological 
sources of evolution in political history. Then history becoming biological will for 
the first time be admitted as a branch of scientific inquiry, and will also for the 
first time provide the requisite training for the statesman. 
If we turn to examine the possibilities of immediate work with regard to 
differential fertility, there are several directions in which investigations can at 
present be carried out with reasonable hope of success. If we confine ourselves to 
the artizan class, wages are a very reasonable measure of capacity, and the corre- 
lation between these and fertility or fecundity* might fairly easily be ascertained. 
Intelligence might be measured by craft classification extending from engineer to 
general labourer. Much may be done, if we pass from the artizan class alone, by 
comparing the size of families of members of various professions, of those following 
mercantile pursuits, of retail traders and of artizans. Further within a narrow 
class like the graduates of the Universities, it ought to be feasible actually to 
tabulate fertility against a scale of intelligence as measured by academic distinction. 
Fertility of sane and insane stocks, of mentally defective and normal stocks, of 
alcoholic and sober parentages can be ascertained, although the task, of course, needs 
careful social inquirers. Lastly grades of physical fitness might well be investi- 
*' Fertility might be measured by nett or gross family when complete, or for marriages of 15 or 20 
years' duration. It would, I think, be possible to measure fecundity by some function of the intervals 
between the births of the first three or four children. 
